When you bring up a new AP it should be seen as pending on both controllers. Then you just manually decide what controller you want the new AP to be on and you accept it on that controller. From then on it will be local to that controller and foreign to the other. There is no way to do this automatically. You can't just fireup your APs and have the controllers figure out which one will take on the based on which one has the fewer APs.
Also something to look out for. When the APs boot up for the first time after being accepted on a controller they download the AP firmware associated with the firmware on the controller. After the firmware is installed they reboot again and they decide what channels to use on their radios at that point. From then on they remember that channel and will not pickup a new channel unless you ask them too. So, it is best to bring the APs up for the first time in their final resting place. Don't bring them up all up 1-2 at a time on your bench. If you do they will all choose the same channel, the one that is best for your bench environment. Then when you power them down and move them to their final resting place they will keep the same channel and you will have lots of co-channel interference. You can always ask the APs to choose a new channel after you bring them up in their final resting place but they will not do that on their own. So it is just best to fire them up for the first time when you mount them.
Do you have NAC in your environment or are you planning to deploy it down the road? If you have NAC and you put every other AP within the same building on a different controller you will have trouble. NAC will lose track of the client iduring the registration process. The best thing to do if you have NAC, or are planning to have add it later, is to have all of the APs within a building on the same controller.
John
