There seem to be some strange comments in this thread. I would say that if you want an easy time of setting this up then absolutely do not mix brands. Just mixing different generations of the same brand can be a problem, let alone getting two very different compute platforms to behave optimally with each other. My advice is if you want more VRAM, stick with AMD and live with the consequences (namely that it has less support than CUDA for many ML tasks beyond LLM). If you now want a CUDA card for that reason, then expect to not be able to share a model between them.

In terms of ComfyUI diffusion models are much less tolerant of mult-GPU setups than LLMs. You would need a special set of "Mult-GPU" nodes just to do anything and those are really designed for putting VAE and embedding models to a separate GPU to the latent space and diffusion model. Splitting the diffusion model itself can be done with something like the DisTorch multi-GPU node but this isn't particularly stable and won't perform nearly as well as a single GPU.

It might be theoretically possible with hours of research on getting an LLM running in one particular configuration with Vulkan. But do yourself a favour and save that time, money and energy doing something you enjoy rather than fighting obscure driver and library conflicts based on random anonymous forums.