It'd have to be something they wouldn't think of.

[ so neither of them can come up with their own code, it'll be too easily broken, since they don't actually know what they have in common with their mirrors. ]

And we'd have to be sure we're not them when we come up with it - [ which is a whole other headache. after all, what's to guarantee he's not talking to her mirror right now? ] seems pretty circular.