If only one of them can leave (once they figure out how to unbar the gate) then the solution is to have Zorian make a memory packet of Zack, leave the time loop, and upload that memory packet into Past/Real World Zack. Of course there is that little question about what will happen to Zorian if he leaves, considering he's a 'fake'.

Too derivative of Shaper V's Time Braid. Besides, that workaround isn't needed. Given that only one can leave and Red Robe has already left, the next arc is obviously going to be about hacking the controls so they can get out despite this prohibition. And as has already been proven, if one leaves but the other doesn't, the loop will presumably stay active since the 'Controller' is still inside, thereby allowing the other to use the same hack to escape as well. Theoretically, they could even use that escape hack to help others get out. If they can figure out how Red Robe started looping in the first place, they could train up lots of people and get all of them out.
Even better, they no longer need to worry about Red Robe's interference, since he's already gone. So they can undergo months or even years of additional training/exploring before they escape, and they'll get out mere moments after Red Robe left.