Yeah, I feel like they said exactly this.

: You do not have to pay
: twice, though it has not been clarified if you can only have your
: "active" profile on one device at a time.

I'm curious about this myself. Hopefully, they won't make it painful to stay signed into both. I plan on keeping my 360, and I don't want to have to recover an account every time (though, I don't even think that's a current requirement, ever since cloud saves).