To answer your question...
We have it setup up for 50/50 right now.However, when we get to the point of adding more APs, we have the foreign controller for more room. In the future, if we have enough APs, using 2 controllers; will the second controller still be able to backup all the APs from the primary controller and still be able to manage all its own APs?
A controller has a max amount of access points it will support locally and in fail over (foreign)
Total APs supported per controller APs supported in standard mode
V2110 496 (248 local/248 foreign) 248 locally configured
C25 100 (50 local/50 foreign) 50 locally configured
C5210 2000 (1000 local/1000 foreign) 1000 locally configured
Licensing is done based on locally configured access points, you don't need licenses for redundancy, it's built in until you reach the max total a controller can support (Total APs supported per controller).