You can resize the Split Screen size, to do so when the apps are in Split Screen where they separate there is a line, in the middle of the line you'll see like a dash line just tap there and move up or down to resize the screen. You can, download app made by Samsung called Good Lock from Galaxy Store to add color to split screen, use without pausing, pop up view and more.
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Thanks NBI. Previously, the window could be adjusted laterally, not just vertically (this was done initially and then you would choose the second app). This gave you the ability to "zoom" your selection. I must have been the only one on the planet to use this feature since I can find no answer on the topic.
You are not alone! I love being able to have split screen or, as Samsung calls it, "Multi Window Function." What I found on the US site was not very clear. But I did locate instrutions with images on the Samsung UK Support site.