I have been thinking about this all day, and I came up with one idea for you to consider. Since I can't think of many trees of the same species to fit your ideal (citrus and dwarf citrus were all I could imagine) I tried to think of trees that look similar but grow to different sizes. You don't say what kinds of trees you like, so I suggest you use the kinds I like :).

Bright green weeping trees.

I think you should plant a Male California Pepper in the big space and a Mayten tree in the small space.

Or a Weeping Willow in the big space with the Mayten in the small one.

I know nothing about roots, so can't address how the pool would be impacted. It's also tough to give suggestions when we don't know the dimensions of the "large" space. It sounds kind of small to me- how big is it? How much pruning would you be willing to do to maintain a tree?

My California Pepper is fifty feet across, so perhaps a fifteen gallon Mayten tree in your small spot and a big boxed one in your large spot would work better. They grow slowly, so the little one should take a while to catch the big one.

Good luck.

Renee

Blue Atlas Cedar for the large tree and perhaps the weeping version as the smaller? There are two mature trees in my neighborhood and they are stunning!

Two Olive trees (fruitless for cleanliness) they can be pruned to any size, one, let it get large, the other, cut it back. You can prune the heck out of them, or not.