I have come to the conclusion that I was not clear enough in my first post. What I was talking about was a wordpress plugin that pretty much disregards wordpress users and uses phpbb’s users tables instead.
So, instead of calling for information from the table called wp_users, wordpress would call for information from the table called phpbb_users.
I think this would be easier than writing a plugin that creates two accounts in two tables. However, I am no php expert, so I may be wrong.