If those two TBv3 cards are going into 'watered down' PCI-e allocation slots in the Windows/PC then high loads of the USB I/O (and/or aggregate non display bandwidth) probably could get flakey. [ Actually not sure how many PC workstation boards fully support two TB cards either as there is a GPIO socket connection. Some boards there is a specified socket suppose to put the TB card into. ]
I have a configuration like this on PC and "flakey" describes the stability of the solution pretty precisely. I did it mostly because I wanted to run 2x LG 5K (LG 27MD5K-B) monitors in full 5K resolution each and the struggle to get there was... Well, it was a very windows-like experience. Long story short -- it's doable - I run 2xDP out from 1 GPU (Radeon 6900XT) into 2x DP In on the motherboard (Asus ProArt Z690) and another 2xDP out (from nVidia RTX 3080) to 2x Mini DP in to ASUS ThunderboltEX 4 expansion card. According to ASUS, such configuration is not supported, as the ThunderboltEX card should have GPIO connection to the mobo, which ProArt Z690 doesn't have. Running Windows 11.
Monitors work, I have some other USB stuff plugged in (USB3 audio interface, USB2 mouse & keyboard, some MIDI controller etc.), but:
1) sometimes one of the monitors is not detected after a cold boot, I have to manually pull out TB plug from the monitor and plug it in back again; usually it's enough to make it detected. And usally happens with nVidia-driven one
2) everything goes haywire if I connect anything running 4K to HDMI outputs of the card, it looks like there's not enough bandwidth
3) sometimes even plugging in additional USB3 device like a pendrive can result in one of the monitor signals going blank
4) sometimes (especially after GPU drivers update on Radeon card) the left and right side of the screen can be vertically shifted against each other. Sometimes monitor gets detected as one running half of the horizontal resolution.
I can't say how many hours I've spend, BIOS updates performed, driver combos tried etc. trying to pinpoint these issues and make things work.
TL;DR -- I don't know about XDR, but running such setup with 5K TB3 monitors on Windows is problematic at best. I think the issue is with either the USB controller on the mobo, not enough PCIe lanes, unsupported combo of hardware, etc. I think it makes a lot of more sense to run it on Mac Pro 2019 -- which I intend to try as soon as I put my hands on one. Good luck OP!
p.s. I'd suggest trying such setup (i.e. MP 2019) on a short distance first and run 3rd TB cable to some hub (Caldigit) instead trying to use USB ports on the XDRs, to avoid bandwidth limitation.