Good scenario, Mark, and one that's often on our minds. Right now, user accounts are unique to an application, so this sort of crossover can't happen. Luckily (in one regard), Joe is probably totally unaware that Singly is there on the backend - he's logging in to AppOne or AppTwo, and their data infrastructure is invisible/irrelevant to him. So for most everybody out there, the apps look like the internet as usual and the promise of a more connected system won't affect their purchase decision.
That said, we do think that sort of shared login—or really a single, accessible repository for your information—is important to the future of data exchange. It's a big part of why The Locker Project was created, and if not for some interesting policy hurdles we ran into, it would probably be playing a larger part right now. We still have the vision, but it's going to take some work and fighting to crack data out of the current, relatively walled ecosystem. The huge expansion we've seen in available APIs and I-own-my-data philosophy is promising.
In short, not yet, but soon, we hope.