[wellpast]

I use IntelliJ with multiple non-nested Maven projects all the time.

I typically have a projects directory under which I have many subfolders containing Maven projects. In IntelliJ I create a single module rooted at my "projects" folder but for this module do not set up any sources. Then it is just a matter of right-clicking any POM and selecting "Add as Maven project..." I can enable/disable these projects at whim.

You can also simply add each module at a time.

I've always been impressed that it "just works". It'll even do in-IDE compilation against latest sources if it detects a snapshot dependency across (even non-nested!) modules/projects.