THACO stops getting better for fighters around level 21. A multicalss fighter/mage can get to 21st level.
One more reason to give something more to single classes: each class must have something special, something others don't have.
You MUST gain something for having sacrificed all other kinds of learnings, by choosing a single class.
Yes, a Multi can reach almost the same level as a pure fighter, but he can't become more than specialized in weapons (unless you hack the game).
That is the spirit, imho. Those are not really "equivalent" 21th levels...
The *only* advantage the multiclass character almost always had was the HLAs.
They still have it, somehow. It is quite unlikely a dualclass will choose HLAs from two different pools, possibly finding interesting combinations yet to discover, with the new stuff

.
In order to gain HLAs in both classes, a dualclass should do sacrifices for the whole game, while a multiclass has this chance for free.
Now I?ve seen others argue that Dual-classing itself is unbalanced but unless you stop it altogether with your mod you have taken a minor balance issue and made it much worse by making this change.
As a matter of fact, I'd be able to remove spells even from dualclassed characters. I've done it already for the S.A. kit: when dualclassed to mage, the Death Field HLA will appear greyed and unpickable. Here it's fitting since S.A. must always be barred from the Necromantic School.
A similar trick could be applied for all dualclasses, in principle, but I'm still against it though, as a general rule.
Ok, this is more a
PnP residue from the old days:
the concept is that Humans "burn" faster, but brighter.
Demi-humans have a longer life, resistances, can learn many things, they generally have a "slow" attitude towards life.
Only Humans will have the burning thirst of power, knowledge, or simply for life, that renders them capable to attain incredible things in a limited time.
I'd like to preserve this impostation: when you dual, you ARE exactly like a single class. Nothing is barred from you (you can become a GrandMaster, if you dual to a fighter...)
Anyway, this issue might be reconsidered if serious balances issues arise.
So far though, I think multiclasses had the Scepter of "Overpowerness". Think about Solaufein. Mixing HLAs from two or three pools is not a joke, it's overkill by itself. We just try to limit some abuses (I won't try to list the nasty tricks that have been discovered by using HLAs of different kinds...)
Is it possible to compromise? Can you, say, limit the F/M to selected Fighter HLAs until he reaches 20th level as a fighter & then make it wide open. Ditto for the mage HLAs until he gets to 18th level there?
Something about levels can be set, in the tables. Don't know what would happen with multis though.
We'll consider this point for further versions, but multis *must* lose something, IMHO.
About Avengers/Fighters: I *think* they'd get the regular Fighter/Druid table (similarly to the behaviour of multiclassed Illusionists).
Sorry, it's not something I can change anyway, the engine decides..