Do you mean a Radeon Pro 580X MPX Module in a Mac Pro? Apple says yes.
https://support.apple.com/en-ca/HT210319
You can connect one to the top Thunderbolt ports and another to the I/O card Thunderbolt ports. They take two DisplayPort 1.4 connections each from the GPU (four total). You might be able to connect a couple other displays to the HDMI 2.0 ports (GPU supports 6 connections total).
Actually, in another post, you said you have a 27 inch iMac 5K. I suppose you mean the iMac (Retina 5K, 27-inch, 2019). The iMac only has one Thunderbolt controller. The support document at
https://support.apple.com/en-ca/HT210437 says it can support 6K so I assume it has a Titan Ridge Thunderbolt 3 controller to support DisplayPort 1.4 (the tech specs at
https://support.apple.com/kb/SP790?locale=en_CA don't have that info).
The R580X doesn't support DSC, so you can only get 6K 60Hz using two DisplayPort connections over Thunderbolt. Each Thunderbolt controller only has two DisplayPort connections, so you can only connect one XDR display per Thunderbolt controller - that means only one XDR for the iMac.
To connect two XDR displays, you would need to make each one use only one DisplayPort connection. There are two options:
A) Use a non-Thunderbolt connection. Maybe a normal USB-C cable? If it connects at Thunderbolt 2 speed of 20 Gbps, then you would be limited to HBR2. What you want is a HBR3 connection. For that you need a USB-C to DisplayPort adapter and a USB-C to DisplayPort bi-directional cable (does the Belkin Charge and Sync cable support HBR3? It might be too long for that or the chip inside won't accept it). Actually, I don't recall anyone being able to get a single HBR3 connection to an XDR display so maybe you'll have to accept HBR2?
B) Maybe two Thunderbolt connections on the same Thunderbolt controller can work:
1) connect a normal display (to use one DisplayPort connection)
2) connect the first XDR (to use the remaining DisplayPort connection)
3) disconnect the normal display (to free it's DisplayPort connection)
4) then if the first XDR doesn't automatically take the freed DisplayPort connection for itself as a second connection, then you should be able to connect the second XDR.
Once you have a single DisplayPort connection working (use the AGDCDiagnose command to verify), then you can find out the max refresh rates that can be obtained.
People have tried using chroma sub sampling on the XDR (using Windows since macOS gives you no control) but the XDR does not accept it. 6bpc is known to work in Windows but macOS doesn't allow it. Therefore, macOS needs to use 8bpc RGB. 10bpc is required for HDR.
Here's some possible refresh rates for different options (using CVT-RB2 timings):
Code:
rate bpc clock 4K 5K 6K
MHz Hz Hz Hz
--------------------------------------
A) HBR2 10 576 65 37 27
B) HBR2 8 720 81 47 34
C) HBR3 10 864 97 56 41
D) HBR2 6 960 107 62 45
E) HBR3 8 1080 120 69 51
F) HBR3 6 1440 157 92 67
You may need to lower refresh rate to allow for some DisplayPort overhead.
Remember, HBR3 probably doesn't work on the XDR and 6bpc only works in Windows and the XDR probably doesn't go higher than 60Hz. I don't remember if people tried 6K at lower pixel clocks (the built-in XDR 6K timings use 1286MHz for all refresh rates).
Of course, the simplest method to connect a second XDR is to use an eGPU.