Ah, I see. No problem. You’d have to switch back and forth in that case or have two of the exact same TVs to compare at the same time. (Some sets do a better or worse job of up-scaling). You’d also have to take into account viewing distance from the TV. At a certain distance it won’t matter, but as you get closer, it matters more and more. There are view distance calculators available online to help with that.
On second thought, If you use the same source 1080p video and lock one in at 1920 x 1080 window and expand the other one to as full screen as possible. The full screen video will be up-scaled so you should be able to compare directly on one set.