The dual class rules were very simple in the LBB, with few frills:
While changing class (for other than elves) is not recommended, the following rule should be applied: In order for men to change class they must have a score of 16 or better in the prime requisite (see below) of the class they wish to change to, and this score must be unmodified. A Cleric with a "strength" of 15, for example, could not become a Fighting-Man. In any event Magic-Users cannot become Clerics and vice-versa.
There are some interesting things about that short statement:
- This is the only place where minimum scores for entry into a class are mentioned; in fact, it's well-known that the LBBs don't require minimum scores of any kind for elves, dwarves, halflings, or single-class human fighters, magic-users, or clerics.
- There is no requirement for the first class, which makes sense given that players might not know they are going to change class until later.
- Clerics and Magic-Users are defined as mutually exclusive, without reason or further explanation. Presumably, their ways of magic are to be considered incompatible, but this is not openly stated.
- How to handle hit dice is missing. I'll have further thoughts on this.
- There are no other restrictions placed on the class-switching character; no experience penalties for using abilities from the previous class, no delays before both classes' abilities are accessible.
- There is no ban on switching back to the first class.
- There isn't even a ban on switching too soon; in theory, a 1st level character can switch classes before reaching 2nd level.
Because the rules are so simple, you can actually eliminate some alternative classes that are easier modeled as a class switch: holy warriors, for example, could be handled as Fighters who switch to Cleric (since no vow against edged weapons is explicitly mentioned in the LBBs, GMs might consider it to be a matter of class training, making holy warriors exempt from normal Cleric restrictions at the price of a 16+ Wisdom requirement.) If you add the Paladin, you can treat it as a sub-class that adds limited clerical ability, but skip the high-level acquisition of Cleric spells, handling that through an actual class switch to Cleric if the player desires.
I prefer this older, simpler approach, and think it can be used as a general model for so-called "multi-classing" by non-human races that's better than the model presented in Supplement I or AD&D. The only by-the-book restriction on switching back and forth between two classes is the need to meet the high prime requisite for both classes; so, a human with Strength and Intelligence of 16+ each is almost like an elf, excluding the ability to cast spells while wearing magic armor, which is unique to elves. What makes an elf different is that magic armor loophole and the absence of any minimum prime requisite requirements: an Elf is a Fighter who can change class to Magic-User and back for free. Either a human Fighter/Magic-User or an Elf can cast spells while wielding a sword.
There's one gap in the rules: how to handle hit dice. Presumably, gaining a hit die is gaining a hit die, regardless of class, but this could make for some odd book-keeping, and it would make Elves pretty powerful. My preference is to use the highest hit dice of both classes. Thus, humans changing classes do have a hit die "freeze" until they earn more hit dice than their previous class, and Elves use their Fighter hit dice unless their M-U level is more than twice their Fighter level.