Is it possible in cycles to render with 2 different GPUs at the same time, or do they have to be the same?
Yes, check my posts …
Mind you, though, that the card with the least amount of VRAM will become the limiting factor. The scene has to fit in the memory of all cards. So, if you have one card with 6 GB VRAM and one with only 2 GB, you will only be able to use 2 GB for rendering.