2 different GPU in one PC for better performance?

Here’s the thing, i have one 1070 ti for two monitors, also i have a 1060 3gb version not plugged yet, i use this pc for gaming, streaming and capture, im playing on 165 hz monitor, so my CPU and GPU are always in high usage to reach 165 FPS when gaming, to other hand on the second monitor is used to watch streams, videos etc and the performance of my 1070 ti is diminshed by this, like 10% or 15%, if i plug the 1060 on the second monitor it will help in performance for my 1070 ti? and for the streaming and capture i know i can use a second video card for this on OBS studio but i have not tried yet, my motherboard and CPU only support dual GPU on PCIE 3.0 X8, run the 1070 ti on PCIE 3.0 x8 will decrese the performance? these are me doubts if someone could help me with this it will very appreciated, Thanks.tweakbox appvalley https://vlc.onl/