**Merging user accounts for zen cart and phpbb3??

I am running zencart 1.3.9f and phpbb3 3.07 pl1. I was wondering if anyone knows of the code or script that would allow a user, once registered on zencart to automatically be registered for the phpbb3 forum, and vice versa? Any thoughts on this? Thanks!!

Probably could just share the user database back and forth, got to be careful with the security keys though.