Actually, that won't work. Any time you do a compass cal, it calibrates all compasses that are plugged in.
The first question is: how can you have a 15 degree error on one that you can solve with a calibration, yet doing that cal would cause a 15 degree error in the other one? If you have a fixed 15 degree error, something else is up.
If you really want to solve this, the correct thing to do is to do a cal of one, export your settings, do a cal of the other, export your settings, then merge them by hand (the mag cal is in the settings). But I'd like to know more about what is going on with your install and cals than to just take you down that path.