It depends on the needs and setup. In practice, you can likely get away with two, but I would never ever try anything new directly on the production server without having tested it on an identical setup (unless you've done it before -- more than once). It's the best way to ensure a positive experience for your end-users.

Yes, stepping into a CMS is an experience. While the setup and initial going is easy, there is always a bunch to learn to make it shine for you and your users. If you've worked with as many of these various CMSes and had to adapt them to work for the demanding needs of several disparate organizations, then you'll find that XOOPS has the best foundation and easiest ability to make it all work well, right, and solidly. You can do the job with any of the CMSes, it's just which one is easier and which one will provide a solid foundation into the future because changing from one to another is even more work.

Happy XOOPSing!