" Permanent Spell (p. 27): "Dispell Magic which are are least twice the level of the caster of the Permanent Spell will negate it, so three Warlocks could negate the spell of a 12th level Wizard...". Notice that the three Warlocks (level 8) are taking the sum of their levels for this consideration (3×8 = 24). "
Never saw that detail - and I don't have my AD&D books with me right now so I can't recall if it survived editions.
Anyway, I do think where this level-stacking may be applicable is not with the arcane casters, but with the divine. Specifically with the turn undead skill, where two clerics working in concert, calling down their god's favors to banish the undead.
Here's the familiar table, for B/X and LL:
Now you take your two clerics, gesticulating, waving their holy symbols, chanting and praying, creating a holy hullabaloo...
I don't care for a straight addition scenario (ex. lvl 4 + lvl 3 = equivalent lvl 7). That seems a bit overpowered to me. I'd probably opt for a +2 or similar bonus roll on the turning die. Alternatively, perhaps pro-rating the summed levels may make for a reasonable summation compromise (1.5x????).
If the played system also has a roll for number of undead turned, add a similar bonus to increase the number turned, destroyed, etc.
Want to add a bit more complexity to the equation? Perhaps the two clerics need to be of the same 'religion' to get the full effect of any bonus or summation, however selected. Two priests of Cuthbert get a full summation. Cuthbert and Vorn get 1.5... Or none - after all gods may be petty and not appreciate the interference from another deity. (EDIT: Jealous gods could impart a penalty!? "How DARE you travel in concert with an infidel in my eyes! Deity dope-slap for you!")
Disclosure - I haven't tried this in a game, so if anyone plays it out, I'd be interested to see how it works out in gameplay, or if another option may be favorable.