XPS 8950, M2 replacement

You can leave the M.2 SSD 512MB as the boot drive, as that is plenty of space for the operating system files. You can add a second M.2 SSD 2TB for files that are accessed often or require fast startup. You can add a SATA SSD 2TB for storage. The M.2 SSDs will be the fastest.

I am not sure where you are seeing a "slot 1" and "slot 2". According to the Specifications document, if you have two M.2 SSD drives, the one that contains the operating system is the "primary drive".

thanks, by slots, I meant board has provision for 2x M.2 cards.

Assume they are labeled so that first is boot drive.

Even though M.2 which I haven't used before, assume each M.2 is still seen and treated as a seperate physical drive right? but much faster as doesn't go through SATA controller?

Again, based on the Specifications document, whichever M.2 drive has the operating system will be the boot drive. I don't see any specification as to which slot that has to be connected. Yes, separate physical drives, and M.2 will be faster than SATA SSD, which will be much faster than a HDD spinner.

Professorwood is correct. It doesn't matter what slot the drive is in.  I cloned my C drive (stock WD SN810 M.2 NVMe) to a firecuda 530  NVMe M.2. I put the Firecuda drive in the empty slot and used a cloning program. Then I formatted the stock WD drive and use it to run other programs.  The PC boots as it should. I did have to switch the BIOS to AHCI/NVMe from the stock RAID setup because the PC kept changing the boot order.

Oh, no issue with locked files when attempting to clone an active OS drive? 
Can I ask what software you used, as seen a few free ones out there.
And M.2 wise any restrictions you're aware of in using non dell branded as considering the  Crucial P5 2TB M.2 2280 NVMe.
Thanks!

no restrictions on M.2 brand . . . get M-key

Macrium Reflect (free) is the cloning software I use

I used Acronis which is not free. Many people have had extremely reliable results with Macrium Reflect.....and it is free, also if you do have some issues, there are many users on this forum that could help you and answer questions. Everything cloned just fine. Like I said, I did have to switch the BIOS to AHCI/NVMe from the factory RAID setting because the PC kept switching the boot order. Switching to AHCI/NVMe involves more than just changing the setting in the BIOS. I would suggest searching the XPS forum as there are a few different threads that thoroughly explain how to do it correctly.

There's a architectural difference between slots shown on image from system manual referenced by ProfessorW00d, those go via different PCIe controllers.
M.2 Slot labeled as (15) is plugged directly into CPU PCIe lane while M.2 slot labeled as (5) goes via PCH/Chipset and potentially can be a subject of some DMI channel bandwidth or PCH own (CPU has different) thermal throttling limitations.
Generally best performing drive (but this might depend on type of workload) better works if plugged into CPU PCIe lane, not PCH (although difference is rather theoretical, in practice it's hardly noticeable anyways).

Also if GPU is plugged into CPU bound 16x PCIe (as opposed to PCH PCIe) then Nvidia's technology "Direct Storage" works better with storage hooked into CPU' PCIe lane (again - due to some possible CPU to PCH DMI channel congestion) since lots of overhead intermediary communication and command cycles are skipped.

Here you go, RAID>AHCI

1. Open Cmd prompt window, run as administrator
   
   
2. Copy-paste this command, which will start Windows in Safe Mode the next time you reboot: bcdedit /set {current} safeboot minimal and press Enter
   
   
3. Restart the computer and enter BIOS setup
   
   
4. Change the SATA operation mode from RAID to AHCI
   
   
5. Save the change and exit Setup and Windows will automatically boot in Safe Mode
   
   
6. Open Cmd again, as in step #1
   
   
7. Copy-paste this command, which will start Windows in Normal Mode the next time you reboot: bcdedit /deletevalue {current} safeboot and press Enter
   
   
8. Reboot and Windows will automatically start with AHCI drivers enabled

Don't forget, if you intend to use the new SSD for storage rather than as boot drive, you'll have to format it.

You can also install some of your apps on the 2nd SSD, even if it's not the boot drive and they'll use that 2nd SSD for storing their output and any temp files, etc.  You can also move Windows default folders onto a different drive to save space on the boot SSD (C:).

Do you have a set of the engineering schematics, or are you following the tracings on the motherboard?

Be mindful that Crucial P5 2TB M.2 2280 NVMe PCIe SSD is a PCI-e 3 drive.  You should look into Crucial P5 PLUS 2TB M.2 2280 NVMe PCIe SSD which is PCI-e 4. if you do matter the speed.

I don't but I'm going with reference design by Intel like shown here:
https://www.intel.com/content/dam/www/central-libraries/us/en/images/prodbrief-z690chipset-blockdiagram-1440x1080.jpg.rendition.intel.web.1920.1080.jpg

Also hwinfo64 can show you (what bus) where peripherals are plugged in (PCH or CPU PCIe controller)

Yes, good point! Difference in speed is quite significant.