I'm not quite clear on your proposed wiring diagram, however I think you're planning to connect both your aircraft's batteries to the Dynon main power input? This would not give you the same level of failsafe as you would have with the Dynon backup battery installed, and depending on how the wiring is connected, may even cause a higher potential for failure. If you can tolerate a failure of the electrical system taking your display offline, you really don't need the Dynon backup battery. You should NOT hook-up any battery to the Skyview display backup battery input except the factory supplied backup battery!
edit-
In re-reading your post, I think I misunderstood your question; are you talking about multiple displays sharing a single dynon back-up battery? If that's the case, then I'm not sure what the results might be. Dynon would have to answer that. One downside is that the battery would only last half as long, another might be that two displays may discharge the battery at a higher rate than the battery can safely support.