*draws diagram*
I mean, two-thirds of this thought, I should have had right off the bat--so, you know, a couple of weeks ago?--but I didn't quite get it until just now, thinking about Jack being all "I've crossed my own timeline" with Torchwood 1901...
Jack's already done that. Because in Spring 2005 or so, there he was in Cardiff with the Doctor, and also chilling out elsewhere in Cardiff, running Torchwood and waiting for his other self to move it along and get out of the timeline so he could finally goddamn contact the Doctor without breaking the universe. And at the very same time, he was also in a drawer in the basement.
Ha.
If he keeps this up, it's eventually going to turn out that half the population of Cardiff at any given time is variously modified/disguised incarnations of Jack.