Hey Dominik,
Did you guys were working on different branches of the same project?
If so, you might wanna have a look on this doc,
https://docs.mendix.com/refguide/version-control/#merging-branches
This will tell you how to merge branches,
Hope it helps!
Hi Dominik,
If one of you two runs Commit (in studio pro), and the other one Update, and than commit. You merged your projects together.
Good luck with it,
Jan