Well, speaking of roleplay use and not necessarily powergaming use (apart that if you are using gestalt characters, you ARE a powergamer
), one thing that I could think of is to choose for each character one class in which he is advancing through adventuring and the other class in which he is learning when he is NOT adventuring. This doesn't mean to change level advancement, you just use the normal rules. Only that each character is explained as learning and improving in class 1 because he goes into action, he fights monsters, he clears dungeons, etc... Instead she improved in class 2 because when she is back in town she is part of a group which study some field, typically a spellcasting class.
So for example, class 1 should be Fighter, Paladin, Ranger, Barbarian, Rogue; class 2 could be Wizard, Sorcerer, Cleric, Druid, Bard. (Monk is a bit in the middle...) So if you want you can have a gestalt campaign where everyone has to choose everytime one type 1 and one type 2.
Whether this is a good idea, it's another thing...