Ratskinner
Adventurer
It didn't punish you much at all until the XP progression leveled out. And by then, the demi-humans were already hitting level limits, and getting pounded that way. At the earlier levels, the exponential progression meant that you could be a 5/5 fighter mage while your human allies were at 6th fighter or 6th mage. You never got more than 2 behind, except perhaps fighter/mage compared to single class thief--or of course if you triple classed. It was usually a good trade--and when it wasn't it was the level caps that did it in (e.g. halfling fighter/thief).
IIRC, in 1e the level caps were a double-whammy, because you still had to pay for your non-progressing class. I don't recall how that worked either way in 2e. Also, at low levels, the lower hp usually meant you were quite vulnerable in your "heaviest" role, especially since you stayed at first level almost twice as long as the rest of the party. Later, you were always behind your compatriots, by a level or two of your already low hp. I saw a lot of Fighter/Mages that ended up being mages who wore armor and gained levels slowly because they were generally too scared to enter the front line and behave Fighterishly. I'm not sure I recall any Mage/Thieves surviving long enough to be relevant. Fighter/thieves had armor issues, and either solution hurt. Of course, as with a lot of Old School stuff, YMMV quite a bit.
Sometimes I'd like to see a version that had 3E/4E unified, cleaned up, balanced classes, but with a 1E progression and multiclassing (i.e. a single XP chart that you had to use in the 1E way to progress, but wide open on choices, and each choice being valuable). One of the nice things about such a system is that you can go as wide or deep as the group wants to go. If you want to grab three levels of cleric instead of your next level of fighter, it might be worth it, even if you are only getting the power of a young 3rd level cleric in the bargain. You do need to work out how hit points and other such things will stack, but that is not insurmountable.
I'm not sure what you're describing here.