You need to put the SSIDs of same name on Same Channel.
Eg. All "Office" APs on Cahnnel 1, All "Guest" APs on Channel 11
This also gives you the ability to roam between buildings without losing connection.
Depending on what type of gear you have, it may be able to "time itself" with the other nodes to avoid any interference that may be present. for instance Motorola Access Points can do this with ease.