Fun with multiclassing (OSR)

I've never been a fan of multiclassing or dual-classing, and I am not sure I could tell you the difference without looking it up. In my games, we usually do race separated from class, BTW.
 
However, I find multiclassing in B/X games could be ridiculously simple: just divide XP evenly, and get the best of each class. I think this is the official rule (or how most people do it).

For example, if you're a fighter/MU with 200,000 XP, you pick the best abilities (spells, saving throws, weapons, HP, etc.) from a fighter and a MU, each with 100,000 XP.
 
Notice that the elf almost works like that already. An elf with 120,000 XP is not that different from a PC that is both a Fighter and a MU with 60,000 XP each. He has dark vision, can hear noises, and has a ridiculously specific immunity to ghoul paralysis, but he stops at level 10, so things balance out.
 
Let's see how many problems this "fix" could solve.

Alex Negrea.

Alex Negrea.

- Level limits. A Halfling can multiclass into thief or fighter, thus transcending his level limit at a significant cost. Each possibility (Halfling, Halfling-thief, Halfling-fighter) has its own strengths and weaknesses. Same for elves and dwarves. The result is similar to a 1978 (!) house rule.
 
- Half-races. Half-elf? No problem, just add an human class to elf. You can combine elves and dwarves to create gnomes, and so on.
 
- Three classes? No problem, divide the XP evenly by three.
 
- Monstrous races. You could have kobold, goblin, and lizardmen NPCs  - any creature that is not remarkably powerful. Just pick a class with a 50% XP tax , get the appropriate powers (usually darkvision, maybe +1 damage), you don't even need to create anything. These are not necessarily balanced - a kobold fighter will always be weaker than a human fighter (although the kobold probably wins in the dark...) - which is fine, as it keeps kobold as weaker humanoids as they are described in the books.
 
- Dual classing - okay, I googled it. So, you could "multiclass" when you're already a 5th level fighter, for example. I see no problem in principle - all your XP goes to your new class until it can match your current XP, and then divide it evenly when both classes reach the same XP. Also, add some downtime and in-game justification for why you became a powerful mage so quickly! This is easier than most archaic, byzantine dual-classing rules.
 
- Balance? This is more or less balanced in most ways I could imagine. It is far from perfect because classes are not balanced to begin with. Notice, you HAVE to keep the XP division even. If you start distributing XP in some weird 75-25 split, for example, you get unbalanced results. 
 
For example, a 14th level MU would become a 12th level MU who is ALSO a 9th level fighter - with his own barony!
 
Of course, this is not meant for infinite XP. Let's try with 1,050,000, for example - the highest in the game AFAICT. This gives us, for example:
 
- A 14th level MU.
- A 10th-level MU who is also an 8th level Halfling. No problems here.
- A 10th-level MU who is also a 11th level fighter. Still has a barony (sigh), but no MU apprentices and doesn't get access to 6th-level spells.
- A 10th-level MU who is also a 12th level cleric. Does seem too powerful (although IMO this is a cleric problem). Probably fixable by using a single spell progression. You still have 5/5/4/4/3 spell slots (like every 12th level cleric), so you could cast four fireballs or cast striking four times - or two of each.
 
Maybe we need some different limit. Maybe 1,200,000 XP, limited to level 8 in your secondary class, disallowing multiple types of stronghold, for example? Could work, thus adding even more options - a MU/fighter distinct from a fighter/MU, for example. 

An MU/elf abandoned other elves and might build a tower one day, but not become an elven leader; while a elf/MU still lives among elves (this redundant combination might be nearly useless, however).

Ability score requirements still apply, etc.

This gives us... more than 30 possible combinations, even if you prohibit some of them? If you use more than seven classes, the sky is the limit.

It is very simple... but I want even simpler.

 
As I've said, I don't really use multiclassing. I've been using feats instead. These are from Old School Feats, each balanced separately to disallow overpowered PCs:
 
2.       Dilettante cleric. You can cast spells and turn undead as a cleric half your level (e.g., if you are level 10 you can turn undead and cast spells as a 5th level cleric), but you’re limited to 3rd level cleric spells. 
3.       Dilettante fighter. You can use any weapon and armor. You can cast spells and even use thief skills in armor, although your ability to climb, move silently and hide is halved in plate armor. 
4.       Dilettante mage. You can cast spells as a magic-user half your level.
5.       Dilettante thief. You have access to all the skills of a thief half your level, including read languages (when you get to level 8).
 
Notice these do not affect saving throws or allow you to build strongholds or create magic items - not even raise dead if you're not a cleric. A dilettante PC remains significantly less powerful than an actual magic-user, fighter, thief, etc. 
 
As far as I can see, this is easier and more balanced than the usual multiclassing rules. You can still use the other feats in the book to create a wizardly elf, a barbaric dwarf, and a halfling sniper. Check it out if you haven't!