The ending seems obvious, I don't see why nobody got it...
I thought it was quite obvious...
At the end, when both Rhondas meet, it proves the theory of desynchronization of both Earths, that they are not the same anymore since the 4 years earlier...
While Rhonda 1 gives John 1 her ticket, Rhonda 2 keeps hers, and then goes and meets herself in Earth 1 (for whatever reasons, curiosity to see herself, whatever), but I don't think there's any mystery here...
Just don't get all the fuss about the ending...
"LIVE LONG AND PROSPER"