I working on a homebrew story and I want the villain to resurrect their Demi-god leader. But I also want the Demi-god leader to be possessing a body so they can be a part of the preparation of the spell to bring them back. I couldn't find anything about using the resurrection spell on a soul that is already possessing another body and if that's possible. Is there anything against this? Is it up to my discretion? Am I overthinking? What do you guys think?

Firstly, if you're the DM, everything is your discretion. It might or might not work by RAW, but really, if you want it to work, then it does. it doesn't matter whether or not RAW supports it. Maybe these cultists found or created a different version of the spell from the one in the book, and it does exactly what you want it to for the story. That's perfectly allowed.

Second, if you're looking for something to let you swap souls around between different bodies, you might check out the spell Magic Jar.

Thank you! This helps a lot!

There is a ton of magic in D&D that are not spells that can be learned. Virtually every cult ritual is an example of such.