I would do it with a slightly diff. focus that Mark suggested
One of the developers is the Master. The other one is the (mmm, lets see, slave is the wrong word, sub-master, sounds confusing... mmm,) NOT the master.
The Master can do whatever he wants to do as far as adding / changing the BO's. This will NOT affect what the other(s) are doing. Obviously the others will have defined tasks (like, you work on these 3 queries, on this VP, etc.) If they need any changes made to the BO's / Relationships, new fields added to RegularUser (which I seem to do constantly), they will convey the request to the Master).
Then, the subs BSV is imported into a NEW application on the Master's system, and Queries, processes, forms are copied over. Should be easy - peasy
Bruce
(oh yea, with a backup BEFORE you ever attempt to merge any of the applications... Lots of backups!)