In //UE4/Release-4.24 cl 11590231 no more than 1 Standalone window will appear regardless of what Number of Players is set to. Currently trying to get that to work to test for regression.
This doesn't seem to be happening in //UE4/Release-4.23 cl 10478456, where the framerates of the focused and unfocused window are not the same, but the difference is much smaller. e.g. 100 fps & 70 fps in 4.23 vs 200 fps & 30 fps in 4.25. For these reasons I am listing this as a regression.
When one window is focused, it displays at max FPS while the other window runs much lower. When neither window has focused they both display at about the same FPS
Both windows run at highest FPS per device hardware
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-97530 in the post.