Hi @ghostship2

What drivers are you using ? Switching between the RTX and GTX should be seamless and you shouldn't see any problems,you can't use multiple GPUs,only single GTX or RTX GPU,if you are using RTX,please try OptiX,at first OptiX will needs to recalculate shaders which is normal and we are seen that in Blender as well when you switch from CUDA to OptiX there is recalculation which shouldn't take more than 1-2 minutes and less (depending on the scene)

You can also try play with Adaptive sampling on GTX GPUs,it works beautifully on GTX GPUs,tested that and you can render as fast as with RTX and OptiX

Freezes not sure if I have encountered,do you have enabled any chance Render in separate process ? Can you try turn off that ? Are you rendering in foreground or in background ?

Thanks,Jura