The easy way is to open two projects at once, copy content from one project, then paste it into the other. (If your project uses referenced event sound modules, make sure that any referenced events are copied across as part of the same multi selection as the events that use them.)
Incidentally, we generally recommend that all users working on the same game project work in a single FMOD Studio project, if possible. Merging projects is certainly possible, but there’s no real reason to do it: There’s no upper limit on how large an FMOD Studio project can be, and our revision control integration makes it possible for multiple users to work on the same project without getting in each others’ way.