Dual classing in AD&D 1e was something that was either totally avoided, used solely in the hopes of building a Bard, or simply totally abused.
My name is Erik, and I was a Dual Class abuser.
Dual classing wasn't easy, until Unearthed Arcana came out with it's empowering alternative stat generation method. Well, that or out group's own alternative empowering stat generation method. What can I say? It was High School ;)
You needed a 15 or better in your original class's prime stat, and a 17 or better in your new class's prime stat(s). Not easy to do by the book, but much easier of the book at UA.
My favorite combo, which I was only able to pull off once, was a Fighter (1 level only)to Illusionist. The reason it worked so well was multifold:
The 1st level fighter max of 10 HP was 2 1/2 times that of a 1st level Illusionist. Even squishy casters need hit points.
Specialized in two handed sword meant a decent amount of damage in melee if needed. +1 hit / +2 damage was substantial when added to the d10 / 3d6 damage of that blade of death.
Upon reaching 2nd level as an Illusionist, Phantom Plate spell became a nice combination with the 2 handed sword.
Between Plate and Sword, few suspected there was an illusionist in the party.
I believe that he made it to level 6 as an Illusionist before the campaign faded. It was fun while it lasted.
As you can see, I was pretty good at "Gaming the Game" back in the day.
Even then I wanted to see rules that explained how you could train for a new class between adventures, when the assumption was that your first class took years of preparation. As far as I remember, there wasn't anything in the rules covering it.