Using multiple graphics cards in Davinci Resolve
The free version of Resolve doesn't support multiple GPUs.  The Studio (paid) version does.  However, combining multiple GPU cards generally doesn't increase VRAM (i.e., if you only have 4GB per card, a pair of cards presents 4GB of VRAM).  If you are thinking about a second card to increase your VRAM, it won't work.

Also, the general story from the Resolve users group forums is that multiple cards are really only helpful if you are using lots of temporal noise reduction (which eats VRAM very quickly). If that's the case, you need both lots of VRAM and lots of GPU compute capacity.