This is semi-close to an upvote. What's holding me back: The prose feels stilted. Sentences that go on for a bit too long; dialogue tags that aren't necessary — adverbs that make otherwise strong sentences feel weaker (eg: "The footsteps seemed to echo a little too much. The walls were a little too bare").

The other major weakness: You open with Carl, we're in Carl's head, but then we move to Jack's head and it becomes Jack's story. Carl serves only one purpose here: He asks the last question, which gives us the final revelation (the twist). His only purpose in this story is to facilitate Jack's story. I feel like you could have made this more effective by starting in Jack's head, with Carl visiting him.

ETA: This is a minor gripe, though. I can see the argument for wanting to stick with Carl's head at the beginning, since it gives us some distance from Jack before entering his head-space. That being said, I feel like Carl is being underused, here; he's a character in someone else's story without any real character of his own. So, really, my complaint here is that the story should either 1) Stay out of his head (to downplay his significance), or 2) Make him a bit stronger and memorable, to set him up as a better foil/contrast to Jack.

All in all, despite these complaints, this is a neat idea. I like the focus on a character study. All the problems I'm pointing out are minor; any of them on their own wouldn't be enough to nudge me to a no-vote. It's all of them in concert that got me there.