I fear a situation where the communities on the respective sites will each claim the question is off-topic and punt it to the other.
For instance, a question about the Android Facebook client is excluded from web apps because it's about an Android app, while it is also excluded from the Android SE site because it's just a client for a web app.
All that will do is serve to exclude and annoy the people who are honestly looking for help and for whom these sites are ostensibly created.
I think a little flexibility and allowance for overlap is needed. Yes, off-topic questions are annoying and diffuse focus, but rigid "no, you need to go here" messages for borderline cases smack all-too-much of government bureaucracy where each department passes the buck to the next.