intergration?

Hi, Ive been trying to figure out ways to intergrate two applications to use a common user information database... example: intergrating phpnuke to work with vbulletin.. so the use doesnt have to re-gister to use each service... any suggestions on how to go about doing this? I have the dbase structure of both the appz, and can alter the code for one of the apps.. not the both... so its a kinda one sided thing.. what would be the best approach for something like this? Regards, Kunal Jhunjhunwala