Question: multiGPU different type

hello there. before i bought another GPU i want to ask several question. now i have RTX 3090
Case :
1. if i bought 5060 16GB, can i used it as main processing unit and rtx 3090 vram as added vram? or the main processing still 3090 and added vram is 5060?
2. if i bought 5090 32gb can i used it as main processing unit and rtx 3090 vram as added vram?
3. which one is worth for money?

