XPS 15 9560 to two external monitors

@jessicaroyale, your system can use MST, and technically that would be a cabling option for you since the Dell display supports it (the LG wouldn't need to explicitly support it if it's at the end of the chain), but the resolutions of your displays are too high to run both of them at their native resolution and refresh rates in that configuration.  The DisplayPort 1.2 signal available out of the USB-C connection simply doesn't offer enough bandwidth for that.  If you had dual 2560x1440/1600 displays, MST would be fine, but the P2715Q is a 4K 60 Hz display, which consumes an entire DisplayPort 1.2 channel all on its own.  If you enable MST on that display, it limits itself to 4K 30 Hz in order to have enough bandwidth left over to pass down the chain to something else, but 30 Hz isn't a satisfactory experience for most people, so keep MST disabled on the Dell display so that you can use 60 Hz.

In terms of how you can run both displays at their native resolutions and refresh rates simultaneously, you have a few options:

- Connect your P2715Q using a USB-C to DisplayPort cable and your LG Ultrawide via HDMI.  Connect the USB-C end of that cable directly to the system, NOT through any sort of adapter like the one you mentioned, for reasons I explain in my next reply below.

- Get a Thunderbolt 3 to Dual DisplayPort adapter and connect each display to that adapter via DisplayPort.  Fyi this adapter is NOT the same as a USB-C DisplayPort MST hub, which looks similar and costs less.  The difference is that by tapping into your system's Thunderbolt 3 capability, this type of adapter gains access to two DisplayPort 1.2 outputs, thereby offering enough bandwidth to drive both displays out of that one connector.  A USB-C DisplayPort MST hub would only tap into a single DisplayPort 1.2 channel, which won't be enough.

- Buy a TB16 dock with a 240W adapter, and connect both displays to the dock.  The P2715Q would have to use either of the DisplayPort outputs, and the LG could use either the remaining DisplayPort output or HDMI.  This setup would also allow both displays to run at their native refresh rates, and you get the added benefit of having everything coming to your system via a single cable -- including power to charge the system -- but obviously it's more expensive than the other solutions.  Note that the power to charge the system is why you'd need the TB16 specifically with the 240W adapter, not the 180W version that's also available.  The 180W version isn't able to allocate enough power to the attached laptop to run the XPS 15 models properly.