I waited over a minute with no result.
So you think if you wait long enough they get into sync automatically? Take another read of the post of PaulS. Serial communication means one bit AFTER the other is transferred. This needs time. First some time is needed for the serial communication from the first Arduino to the XBee, the XBee has to modulate these bits onto the radio signal, the receiving XBee has to receive all bits, demodulate them and send them to the second Arduino. So you have three serial lines each loosing some time, and every hop inbetween adds some delay because the data has to be processed, even if the algorithm is simple it needs some time. I'd always expect them to be out of sync and never to be in sync.