With all due respect I think you might be a little confused as to what I am saying is my situation, be it my fault or not, it really doesn't matter.
Basically, while working within the program (3DS MAX) I can be rendering a scene (using the 2080tis with the render engine of choice) and still maintain good viewport performance within the program itself. Essentially meaning I could work with limited downtime of restarting rendering but also gain increased response time. In the most basic terms I can think of, the 2080tis would be acting like a separate machine to render the image, while the 980 would be like your normal windows/display device.
With regards to the gaming side. Because I am not in a position to yet justify a second dedicated workstation/gaming machine I'd use the 2080tis to play games. However I believe it must be driver related that when the 980 was still in the machine, even though not being used it would give me awful performance in game.
I hope that makes sense? You probably don't care but I still felt like I should try and explain! :)