I guess I should have mentioned what I was really trying to do from the start.

So, I have 2 monitors, one above the other. Same resolution (both 4K) but different size. Bottom one is 17" and top one is 27". I'm trying to have a wallpaper span across both but of course the top one is wider so it doesn't align with the bottom one.

I wanted to somehow turn it into a T-shaped wallpaper where the top monitor simply has extra content on the left and right side, while the central part of it perfectly aligns with the bottom screen.

So I thought if I can crop wallpapers, I can display the same wallpaper twice on both monitors (without grouping), but crop a wider chunk for my top monitor, and a thinner one for the bottom monitor, so they align, and then just stretch them vertically if necessary.

With your suggestion, I can vertically align the wallpaper in the top monitor to the bottom one, but then I can only have the entire wallpaper twice on both monitors, while I would still need to cut the bottom half part on the top monitor and the top half on the bottom monitor, so that the image doesn't repeat.

Sorry I guess this is hard to visualize by reading it. If it's necessary I can post a pic.