I have a 5090, once you start splitting weights/cache amongst devices or start having to rely on unified memory you quickly enter the land of diminishing returns and tradeoffs around cost, power consumption and speed. I'd just go for whatever the cheapest single device that will fit your chosen model weights + context at the speed and price you are comfortable with, which for most folks will probably be a single GPU and a model in the 30b neighborhood (qwen/glimmer)