I think it should be fairly feasible using the REPLACE INTO function of MySQL (
REPLACE INTO)
So effectively you will be replacing updated information from one forum to another. The question is which will be your master forum and which your child?
One way to do it is to have an intermediate table in which you will replace into from forum 1 and then replace into from forum 2. Once that is done you will replace into forum 1 and forum 2 from the temp table.
This method would synchronize both user tables and take care of the registrations in forum 2 to be present in forum 1.
I hope this helps.