The easiest way to do this is to keep one companion unused until you reach level 12 (I used Minsc). Once you reach level 12 have them join your party and save the game just in case you make a mistake. Then you simply add one level for each class. If you accidentally take 2 levels in a class reload the save you made and start over.
Note - You need to be playing in Balanced or Higher in order to multi-class.