it really depends upon what type of integration you are talking about.
there could be something as simple as .htaccess, which has been done
or
it could be that the sites are sharing some sort of user DB (mysql).
and of course there are other forms of integration. in order to better understand what you'd need -- it would probably need more description from your side.
one thing that is always a little buggerish, is how do you handle new signups? does one side signal the other to auto add to the DB ? or do they share a common DB ?
how would deleting members happen?
all that stuff.. sooo it's more than a small ubbdev mod (in most cases) -- the docuwiki integration is an exception, where the wiki has an authentication api that is ready set to go for integration