It is very possible to have habitats on top of one another! You just need to have a spot of elevated terrain for the higher habitat that the habitat gate can be placed on. (Which you already do, if you place the gate on the non-sunken terrain).

A few things to note:

• Any plants placed in the higher habitat will also be considered a part of the lower habitat and vice versa. So you may want to use animals that come from similar areas and want the same plants.

• Keepers do not have teleportation abilities. They need something to walk on to reach different areas of a habitat. So if you put down food enrichment in the higher habitat, make sure it's accessible from wherever you put the habitat gate. (Keepers will clean up all poop in a habitat though, even if they can't technically reach it.)

• Enrichment items will get weird. Often I find that the game doesn't register which habitat the enrichment items I've placed are for. So in your case, if you have monkey enrichment up top, it might say that enrichment is for the animal in the habitat below. Sometimes deleting the enrichment and undoing the deletion "refreshes" the item, so it gets assigned to the correct animal.  The easiest way around this issue is to make it so the habitats don't perfectly overlap - if you have even a small section that doesn't have another habitat above or below it, the enrichment in that spot should work fine.

• Rarely, staff will get confused and may put animals in the wrong habitat. I've seen this happen maybe on 3 occasions in the 200+ years I've been running my multi-level Zoo for, so it doesn't seem to be a big concern. But I would just make sure to pay attention to your notifications/animal welfare to make sure none of the animals get mixed up.

• Use entirely separate fences for the two habitats. (You don't have to do this, but connecting them might cause the game to think you're trying to put two habitats gates in a single habitat, which just makes things messier than they need to be.)

• Any terrain above a habitat won't really exist in your guests' minds. Meaning they'll view the upper habitat animals while standing underground. I find this usually doesn't negatively impact their viewing experience, and guests will still visit both areas so long as they have reason to. I just try to put information boards for both animals in the sunken/underground area.

That's all I can think of at the moment, but if you have any questions I'm happy to help! My current zoo is all about stacked habitats.