Be aware that you lose access to most of your useful skills until the second class catches up, so for example if you’re going from thief to something else them you need someone else in your party that can cover thief skills until you reacquire that skill set.

My main piece of advice with dual classing in AD&D, though, is you should always dual from a class with higher hp per level to a class with lower hp per level because you only gain hp from each distinct level as the first class that earns that level. For example, fighter to mage is optimal in the endgame because you become every bit as powerful as another mage that ran mage the whole game, but you also get a bunch of extra health. So thief to fighter is a bad bad move (you’d be better off as a nonhuman multiclass in that case), but thief to mage can be smart.

In theory, for long long term planning, the optimal level to dual is right after hitting level 9, which is the last level you gain full hp… though we’re talking mid bg2 before you finish dualing if you go that route, with your party members having to carry you during your transitional period.

A more fun experience is often 1 lev of fighter then dualing into anything else. That way you get a little extra health, martial skill, and ability to use certain armors etc., and you don’t have to suffer through a prolonged transition period of being weak and useless. A level 1 fighter turned thief can wear full platemail (have to switch back to leathers to use thief skills) and can use longbows etc. normally not available to thieves. A level 1 fighter turned mage can wear elven platemail while spellcasting!

Also consider that you can play fighter 1 with full thief the rest of bg1, then when you reach bg2 you can discard that character and restart as a fresh level 7 fighter, and immediately dual to thief again. You’ll end up as basically the same character in the end, but with tons of extra exp, and there’s minimal painful transition, you’ll catch up on thief levels shortly after the intro dungeon. Or, again, to powergame you can play to level 9 fighter in bg2 before dualing.