yes...

@jphughan 

That response was well thought out and helpful. I am having a similar situation and wonder if you have any thoughts.

I have a brand new 7390 2 in 1. I updated all the drivers drivers and BIOS as soon as I got it.

I also have a USB C hub, which I have used just fine on my 2017 HP Spectre. The hub has various ports on it, including 2 HDMI ports and a VGA port. 

I had been using the hub to connect via HDMI to two identical Asus monitors, as an extended desktop. Again, this works without issue on my HP Spectre.

When I plug the hub into the Dell, however, the monitors are recognized as 1. In my device manager, it shows only one external display. So the second external monitor is worthless. Any thoughts on what is going on or how to alleviate it? I guess I can get a USB c to HDMI adapter for the other USB c port. But that seems like a stupid accommodation for a brand new, nearly $2000 computer.

I am scratching my head.

@Jwhite326, that's very strange.  Normally I would have said that the overwhelming majority of USB-C "mini-dock" devices like you're describing don't support simultaneous independent displays, even if they offer multiple outputs.  They instead only support using one at a time or mirroring the same signal.  But you said you've achieved multiple independent displays both attached through that same device on another system.  That capability relies on both the host system and device supporting DisplayPort MST.  USB-C ports send video as DisplayPort, even if a cable/adapter incorporates a converter chip to switch it to HDMI, and DisplayPort is the only video output type that supports multiple independent displays from the same system output.  Based on your experience with the Spectre, the device supports it.  But the XPS 13 also certainly supports it, and it has for generations now.  I myself had an XPS 15 9530 from late 2013 that supported this through its actual Mini-DisplayPort output.

Just to test one quick possibility, does the hub behave the same way regardless of which USB-C port you use?  I remember helping someone recently who found that they could only connect multiple displays to their XPS 13 when both of them were connected to the TB3 ports on the left.  Having one on the left and the other on the non-TB USB-C port on the right didn't work.  I'm not sure why that was a problem at a technical level, but I'm wondering if that hub might behave differently on another port.

Otherwise, since you've already updated the BIOS and drivers, my only other long shot hypothesis here is that if the hub relies on DisplayPort Alt Mode (native USB-C video) for one output but DisplayLink "indirect display" technology for the other, you'd need DisplayLink software installed to run the second one.  The Dell D6000 dock uses this type of hybrid approach to support triple display setups, for example.  Do you have something like that installed on the Spectre?  If not, I don't have an immediate suggestion here, other than a theory that it might be a bug in some driver/firmware that you're currently running.  Ironically a few weeks ago I was helping someone with an XPS 13 that wasn't able to run a 5120x1440 display at its native resolution even though it should've been able to, and the problem turned out to be that the latest Intel Graphics driver release at the time had introduced a bug that broke that resolution, and the fix was to roll back to an earlier release.

If you had another way to test your system's DisplayPort MST functionality -- such as a USB-C dock, MST hub, or daisy-chainable displays -- that would be an interesting test.  But apart from that, I'll post again if I think of anything, but I'm afraid I don't have any suggested fixes right now, or even a theory I'm reasonably confident in.  Sorry!

Out of curiosity, do you have a link to the specific hub you're using?  I might want an MST-capable "mini-dock".  Someone mentioned one in another thread about a week ago, but it had so many reviews citing concerns about overheating, short lifespan, and unreliable performance that I decided not to get it.  I don't have a pressing need for one anyway, but if there's a good one out there, it might be a nice thing to have in my tech toolbox.

@jphughan 

Thanks. Yes, I have tested it with both USB C ports. And somewhere I read about turning the cable over. I tested that too, but to no avail.

I did install the Display Link software on the Dell, as I read about that too somewhere. It didn't make a difference.

TBH, I don't know what was on the HP. It just "worked" when I connected the Hub to it. The Hub also worked with my work laptop, which is a Lenovo Miix something or other.

It allows pass-thru charging too, which is handy with the Dell and Spectre.

Anyway, this is what I bought: https://www.amazon.com/dp/B082D17FWX?ref=ppx_pop_mob_ap_share

FWIW, one other person I saw online had this issue on the 7390 2 in 1, back in late 2019. I think the conclusion was that it is a driver issue. It is just a bit frustrating that they haven't resolved such a basic feature in the five or six months this thing has been out.

Thanks for your time.

@Jwhite326  Hmm, that might be the one I saw before, not sure.  Anyhow, the adapter itself wouldn't require any drivers for video output other than the Intel Graphics drivers that would already be installed, since that's all it takes to use DisplayPort MST as this adapter would require for multiple independent display output, and that definitely isn't completely broken on the 7390, otherwise these systems wouldn't be able to run multiple independent displays using devices like Dell's own WD19 dock, which relies on the same technology.  A bug in the Intel Graphics drivers is a possibility, but if those other systems are running Intel Graphics, then that seems less likely unless it's a bug particular to a certain release or maybe even something that only affects certain generations of the Intel GPU.  I have a feeling this might turn out to be an interoperability issue with the specific combination of this system and adapter that shouldn't exist but somehow does.  They do happen.  For example, a while ago it was discovered that when the XPS 13 9350 is connected to Dell USB-C displays using a regular USB-C cable, no video signal is transmitted.  This is true even though a) the XPS 13 9350 can work with other USB-C displays just fine, b) those Dell displays can work with other USB-C video sources just fine, and c) the XPS 13 9350 can work just fine with those Dell displays when using a USB-C to DisplayPort cable.  It's a very narrow failure condition, but it was experienced by multiple people.  I got it escalated internally within Dell, and they eventually published a KB article about it.  Unfortunately the KB didn't explain underlying cause, and as for a fix, Dell's "solution" in the KB was, "The XPS 13 9350 is end-of-life at this point, so this won't be fixed, but you can work around it by buying a docking station."

Of course even if something similar is happening here, it doesn't shed any light on which side of this equation has the bug that triggers the interoperability problem, and even that wouldn't change the bottom line that it doesn't work even though it should.  Sorry!

I am having the similar situation. Currently using Dell XPS 13 2-in-1 7390. Am planning to get 2 Dell USB-C 4K Monitors (Model P2721Q).

Can this laptop supports these 2 monitors P2721Q 4K 60Hz, without using any docks or dongles?

Example, power to monitor, USB-C from monitor to laptop. What about another monitor?

Thanks in advance for the helps.

@Arthur Of Camelot You should be able to connect both of those displays directly to the system using the USB-C cables that should come with those displays.

@Arthur Of Camelot  Happy to help.  Answers below:

• I haven't tested this personally, but yes I would expect 4K 60 Hz to work on both displays when they are each connected directly to the system via separate USB-C ports.  The XPS 13 7390 2-in-1 supports DisplayPort 1.4 over USB-C, which makes it possible to run 4K 60 Hz and USB 3.x simultaneously over USB-C.  And since XPS 13 systems for a while now have been able to run 4K 60 Hz over TB3, the system should be able to run them separately via two independent USB-C channels.  Again, I haven't tested this personally, but I would expect it to work.
• You don't have to worry about disabling USB PD.  When the system is already drawing the amount of power it needs from a source, it will simply ignore other power sources.  USB-C power sources can't "force" power to the system.  Power only flows after a USB PD negotiation.  Very basically, the source advertises the power levels it can provide, and the system can choose what it wants, if anything.  If the system is already receiving its required power level from an existing source, it won't request power from the secondary source.
• Yes, if you use USB-C connections from both displays to the system, then the USB-C cable will carry video, USB data, and power (from whichever display it decides to draw from).

@jphughan , thanks for the prompt reply.

Last questions:

1) Using USB-C cables for both monitors, both will achieve 4K resolution at 60Hz? Am using the 2in1 7390 4K screen specification laptop.

2) USB-C comes with power delivery, for the first monitor, it will power the laptop. Is there a toggle to disable power delivery from the second monitor to the laptop? Or is this automatically disable for second monitor. 

3) I suppose the USB A from both monitors can be used to connect the keyboard, mice donggle, USB A harddisk, thumb drive etc, correct me if my understanding is incorrect.

This is a great forum, a lot of good info for supports or buying decisions.

You have to put your laptop on docking station. I don't know how it work you can do some reseach but all i know docking station allow for dual display setup