So could be missing something here, and I think we can do this via actions, but I think all you need to do is use the Groups Selected = True on both sheets. You can ether set it up again in the "compare groups" sheet (as it is driven by a parameter that is global), or you can choose the option of "apply to selected sheets" and then click on the compare groups.
Hope that does the trick, makes sense, and does what you need. If not let me know and I can look at another way of doing it.
You may also be interested in reviewing Simon's great presentation in the most recent Think Data Thursday