You guys seem to be missing some major points:
1. The FJ world are desperately short of JO navs so where would all these prospective exchange guys come from.
2. If there were any to spare it is unlikely that your average FJ nav would see an AAC exchange on the Apache as much of an 'upgrade' from what he's doing already, and as an officer he'd probably only get a few 100 hrs out of a tour anyway.
3. If he'd have to do a pilot's course then he'd probably want something fast and pointy out of it as, historically, although many have 'failed to make the grade' - 64AV8or's words- the vast majority of ex FJ navs who do pilot crossover training end up back on their original steeds and many go to the single seat world. Not all navigators are failed pilots 64.
In summary then, in AAC hands it's not as attractive a job as you all think it is.
BTW, I'm intrigued, why does it need only 2 pilots? Why not 3 in case the other 2 get it.