I think we're making a very big deal of something that doesn't need to be.
- If something is on topic there, it can stay there.
- If something is on topic here, it can stay here.
- If something is off topic there, but on topic here, we migrate (or flag until migration paths exist) just like any other SE site
- If something is off topic here, but on topic there, we migrate (or flat, until migration paths exist) just like any other SE site
As Stack Exchange grows, site overlap is inevitable and ultimately not a bad thing. Our responsibility is to determine what is and is not on topic on this site without regard to what is or is not on topic on another.
If it arises that a staggeringly high percentage of questions (50+ %) would be on topic on another site, that's a different story. It's also something that will be blatantly apparent and not something we or anyone else would really need to investigate. I don't think we're anywhere close to that, and neither are they. Until we reach that point, I think we're making much ado about nothing.
Edit:
I've been thinking more about this, and have some thoughts to lay out.
A question like "What can I do to rebound better [in basketball]" is on topic here. A question like "What can I do to jump higher" is on topic on fitness. I think these two things are cut and dry. So is the first question on topic on fitness? And is the second question on topic here? I think answering those two questions will lead us to the our goal of understanding scope.