Later, when Jack was on the Doctor's TARDIS again gallivanting around the universe (and a little less immortal: he'd gotten the Doctor to pour a bunch of energy back out, so he could sleep now, had more fun with booze, and got a limited number of extra lives, like a Time Lord or a character in a video game) they finally made a stop off at Rose's new place, just as soon as they figured out how.
They'd both expected her to be dating. Rose, after all, was a quick-witted young woman, the sort to pine very intensely and very abruptly and get over it. The long period of time they spent figuring out a safe universe-hopping technology had been punctuated with hushed conversations: ("What if she has kids?" "Then we tell her we're just stopping by for tea, make an eighteen-year time-jump, and she'll be glad to see us.")
Jack just hadn't figured she'd be dating him. (At least that answered the "still open to possibilities?" question.)
"Yeah, he works for Torchwood here."
Totally different lifepaths, but they'd somehow independently wound up in the same (sort of) place. Like those eerie twin studies.
After some how'd'ya'dos, Rose left to go fetch a cup of coffee and the Doctor went along to speak with her alone, leaving Jack with Jack.
Jack looked at "himself" speculatively. The other him was leering. Probably the same facial expression, really.
"So--" they both started at once, and grinned.
He let the other version take the initiative. Younger, and more impatient. Jack could wait his turn.
"So, do we do what everyone's expecting us to do?" said other Jack.
"I dunno. Why? It would be like masturbating, don't you think? I mean, not that I ever objected to a good circle jerk, but... I need a little more excitement."
He looked at other Jack some more, and speculated, some, about various physiological similarities. This might not be so bad, but he still needed a little extra frisson to sell him on it.
"Unless you learned a lot of new talents, you've never sucked your own cock."