It might make a little difference if the gpu performance is close. If you have one more powerful and more memory GPU compared to a second, use the better one for the UI but not as UI only. If you have a separate UI GPU, that's something we generally don't favor now, whereas we did 10 years ago. The cards with extra memory waste that memory. If you have GPUs with difference memory capacity, the lowest capacity will define how big an image and the type of processing before its runs out of memory. If you have mis matched GPUs, the slowest GPU will define how fast the system can process. If you have a GPU in the system but its not selected in preferences, its just a waste of space.
Generally it doesn't share memory to other cards.
Resolve uses the GPU memory on every card it has in the system that is selected for Resolve use.
