If you have access to two machines try it that way, when your playing in two edditor windows it only focuses one of them at full fps, the others often appear slightly like a slide show… this can be somewhat overcome by finding the setting thats called something like “run each pie window as its own process”
So if you can test with your friend is probably the best way to see if its actually a problem or just unreal trying to run two games at once…
You can notice this happening sometimes outside of unreal as well, if your watching fullscreen netflix or youtube and a game that your playing sometimes the video seems to be behind a bit or skip some frames