different GPU's question

Total noob regarding GPU rendering, are there any drawbacks using two different GPUs , for example geforce 770 and geforce 1080Ti ? Or it’s pointless and better get rid of old one?

Total amount of usable memory will be the smallest memory that any of the GPUs have. In this scenario you will be limited of the memory of the 770.
You can dedicate it for display only, so the OS uses it for UI and the 1080ti for rendering.

Best,
Blago.