OD&D Elf as Multiclass Template

This started off as one of those weird shower thoughts, but the idea has stuck with me through the morning so I might as well post it, since I've got a bit of time this afternoon. 

I've always found the OD&D Elf to be odd, and I've always been curious about how they play, but the only two times I've played OD&D rules, I played a Fighter and a Thief (Greyhawk supplement, obviously). If by some odd chance you don't know, if you were to play an Elf in OD&D, your character basically has two character sheets, one as a Fighter and one as a Magic-User. I don't know if people actually used two separate sheets, as ability scores, equipment, and so on doesn't change. But each time the session starts, you would decide to play as either a Fighter or a Magic-User, and your character has those abilities but not the other for that session. 

I've had a few posts from time to time about variant multiclassing rules (there may be more, but if so I don't remember how I tagged them). This is yet another thought experiment post, not something I'm seriously considering applying...at least not to my current games. 

Idea 1: Expanding Potential Class Combinations

This is a simple idea. If the Elf can be a Shrödinger's Fighter/Magic-User, why not let the Halfling be a similar Fighter/Thief? At the start of the adventure, select which class abilities you will use this time. You can't rely on the other abilities this session. All XP earned this session goes to that class. 

This is not too complicated, although any multiclass demihumans (or humans if the DM allows) would need to have two sets of abilities, hit points, saving throws, etc. on their sheets (or 2 sheets). One bonus is that it allows players in groups with more casual attendance to pick the class to run that session that will most benefit the party. Mary didn't show up with her Magic-user? OK, my gnome will be an Illusionist this session.

Maybe give them the perk that once one of their classes is maxed out, as long as the other class is higher level, they get both sets of abilities simultaneously from that point on (like the BX/BECMI Elf class, or standard AD&D multiclasses). 

Idea 2: Simultaneous Ability, Separated Experience

This idea would be that as an OD&D Elf (or any other multiclass combination allowed), your character acts simultaneously as both classes, as with the Basic Elf class or AD&D multiclassing rules. But you have separate experience tracks for each class. At the start of an adventure, the player would have to decide which of their classes will gain the experience from that session. This allows players to control the pace of each class's advancement to an extent. You never know just how much XP you're going to get, after all. And I'd be sticking to the "no more than one level advanced at a time" rule for this.

One difference I'd use with this system, aside from the above idea of not dividing XP evenly into the classes, is that hit points would also not be averaged. You'd get the full HP for each level in each class. Yes, that would give multiclass demihumans an advantage in the hit point area eventually. This being the case, I'd remove level limits for single-class demihumans. 

Idea 3: True Shrödinger's Multiclass

This is the weirdest of the ideas. OD&D Elf (or other similar multiclass character), but the player doesn't get to decide which class to use, or which gets the XP, until they use an ability from one of those classes. So if you get in a fight and use a sword, you're a Fighter this session and Fighter gets XP. If you cast a spell first, you're a Magic-user and MU gets the XP. Similarly if you're a Fighter/Thief, if you pick a lock, you're a Thief for the rest of the session. If you use your better Fighter saving throw or hit point total first, you're a Fighter. 

Obviously, for some combinations, the triggers will be subtle. Thieves can use many (or all) Fighter weapons. How do you know they're acting  as a Fighter? Does putting on a suit of plate armor trigger you as a Fighter? What about Fighter/Clerics? Both can wear the same armors, and Fighters can fight with blunt weapons if they choose. How do we know you're a Fighter and not a Cleric if you don't cast a spell or turn undead? 

DMs and players would need to work out a lot of issues with this idea. But I think it might be kind of fun to try some time.