Hi,
I have in my machine 2x RTX Graphics cards 1x RTX 3090 and the other ones is a RTX 2080 TI, my problem is when i choose only 1x the RTX 3090 all works normal and don’t crash or freeze anything when I render or realtime preview in the viewport but if I select the 2x in the preferences i can´t render using realtime in viewport it crashes the blender or gets freezed and i need to shut down the software!!! why i only can use 1 GPU?
thks
What power supply do you have and how old is it?
My power is a 1450 Watt Chieftec, the problem is not related with power supply!! I use both in 3dsmax with octane and fstorm and with overclock and all runs perfect and smooth
Unfortunately, i don’t know what else it could be. You could try running one card at a time or alternating between Cuda and RTX and seeing if it works in those configurations. but it does seem like a bug in the software rather than a hardware problem.
Are you using CUDA or Optix? Check your drivers as well. I’m using 2x RTX 2070 with no issues.
I already tested the 2 ways and drivers are the last ones, but only works when i only use one if the 2 are choosed as for CUDA or Optix when run render by GPU in viewport it crashes or get freezed and in ends will crash also!! it seems that only possible to use 1 at time 
Which Blender version(s) have you tried?
Blender 2.91