So, my secondary display's maximum resolution is 1920 x 1080. My primary display requires 125% scaling. Since 1920x1080 100% is perfect for the second display, you suggest I change the resolution, since I can't change the scaling. In that case, how do I change the resolution of this display to 2400x1350 so it will scale back to 1920x1080 once the 25% is applied? I don't know how to change the resolution on a display to a value higher than its maximum native resolution, though I know how to set it lower.
Well, this won't be a solution for everyone... and it isn't ideal for me, but it is better than what I had before.
I'm able to keep my primary display 28" display at 3840x2160 with 125% scaling (exactly what I want on this display). For my two secondary displays, 1920x1080 and 1680x1050 I was able to use my NVIDIA Control panel to create the resolutions of 2400x1350 and 2100x1312 respectively. So Windows thinks the display is larger than it is, and then shrinks it down to actual size. So it is _sort of_ like 100% scaling; or no scaling at all. It seems the pixels are still a little blurry, but since these are my secondary displays where I just drag a web page to refer to or something, the slightly blurry screen is OK, and I have all the screen real-estate I had before.
I'm still waiting for when Microsoft eventually realizes that everyone needs to be able to control scaling on a display-by-display basis just like they do resolution, but for now I'll live with this kludgy workaround. (And of course, folks without the NVIDIA control panel won't be able to do this at all.)