Anyone have experience of mixing nvidia and amd gpus with llama.cpp? Is it stable?

I currently have 2 5090s in one system for ai using a proart 870xe and am debating selling a 5090 and replacing it with 2 amd 9700 pro cards for vram to run qwen 122b easier than offload to cpu and that new nvidia model. I'm not too bothered about the speed as along as it doesnt slow down too much. wondering if its stable and how much difference Vulkan is over pure Nvidia. When I tested the 2 5090 with a 5070ti from partners gaming pc i got like 80 tokens a sec. Im aware it might drop to like 50 with this setup but thats still decent I think.