While this question is several months old and you’ve probably figured something out already, I figured it was worth answering anyway. This is possible to do using the WP Multi Network Plugin. WordPress has the underlying structure to accomplish this, but no accompanying UI — this plugin merely offers that interface.
Some things are not completely shared across Networks, and I believe Users are one such example. If I remember right, users can span multiple networks, but it needs to be done by a super admin. Anyway, play around with it, you’ll get it working how you want!
Cheers~