Hi gm,
You can assign primary and secondary controllers to specific access points, thus making them use whichever controller you want them to use. Go to the Wireless tab in the 2106 GUI, then click on an access point. Scroll down and enter in the names of your two controllers as primary and secondary. Load-balance them as you see fit.
Once you've applied the configuration, you'll need to reset the access point to make it attach to the other controller (if necessary). You can find a button to do this on the same screen, just scroll all the way to the bottom to find it.
I hope that helps! Feel free to ask if you have any more questions about this.
Jeff