FPS dropping DRASTICALLY with 2 GPU

[VjLuper]

Hi everyone!
I have a PC with two A5000 GPUs (I know it's not the ideal setup, but it's too late to change the system now).
On the first GPU I have the main monitor + 3×4K outputs. On the second GPU I’m running 4×4K outputs.

As expected, performance drops heavily when I start using the outputs on the second GPU! It goes all the way down to around 15 FPS.
Now, I understand that the bottleneck is the frame transfer from GPU1 → PCIe → GPU2.

Do you have any suggestions on how to improve performance?

For example:
Is there any way to tell Resolume to use the second GPU as the main rendering GPU, so that there’s less traffic going over PCIe?
(My main monitor is only 1080p, not 4K, so it would reduce the load.)

Or:
If I replace the two A5000s with two RTX 5090s (kinda faster cards), would that help in any meaningful way?

Thanks!

[Sanial]

You can set the default render card in the nvidia control panel. You can do this globally or per application.