You actually have to click the wooden beam thing to get across the roof both times (and not just click the arrow at the top of the screen that appears once you've used the zip line once) to get the Zip medal. Logic seems broken as logically how are you crossing back over if not by using the zip line? One would assume by clicking that arrow to get across that he does get back across via zip line, but you don't get the medal if you only use the arrow.
Anyway just a little thing to help anyone having trouble with that medal.