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There are two approaches to being a multiclassed character in my campaign.
(1) The Paladin Approach.
You have a primary class and a secondary class, and you advance in the secondary class for free, at 1/3 of the rate of your primary class. That's just how paladins and avengers advance as clerics. The downside is that there's a social cost (paladins and avengers are always servants of their church and can never rise in the theocracy, while at the same time they cannot owe fealty to a landowning noble, so they can't rise socially as fighters either).
That same approach can be applied anywhere else too, so I've seen mage-knights in the borders between the two empires on the Isle of Dawn (a fighter who owes fealty to a mage, never gaining respect or social status as a mage, also not being able to don armour, so never being respected as a warrior either), druidic knights (can't have metal by their skin, and always subservient to one of the Council of Druids), cleric thieves (who, except in the church of Asterius, will always be viewed with either contempt or suspicion by their churches, ruining their chances of advancing in the church heirarchy, and limiting the nature of the favours they can ask of their churches) and a mage cleric, which sounds like a soft option, but said character was constantly hounded by his church to take a more active role, he was expected to use his mage powers for the good of his church, and was also often accused of necromancy. Hardly surpsising when you think about it.
Such characters can advance to level 36 in their prime class, at which point they would be level 12 in their second class.
XP, HD, attacks and weapon mastery is as for primary class. XP bonus for prime requisite of primary class.
A character with a primary character class can, theoretically, become a two class character by this means at any time. It ain't easy, though.
(2) The Double Barrelled Approach.
You have two classes. You advance equally in both until reaching level 12, then subsequently you choose which level you'll go up each time (one or the other), having a total of 24 levels to go to get up to a final 36. The experience point total needed to go up is equivalent to an average of the two totals needed for the classes, plus 15%.
That sounds like an easy option, and using the XP advancement suggestions from, say, AD&D 2nd ed it would be, but in classic a large part of your XP comes from character play and fulfilling the character class role. For a two-class character the problem is that you have to fulfil -both- to get the bonuses, so practically you end up with less, and you'll lag behind the rest of the party till reaching 12th level, typically by two or three levels, at which point you can pick one or the other each level and advance as one at a time.
This approach I've seen for mage assassins and cleric thieves, two popular combinations, as well as fighter mages, thief mages, and fighter thieves (in my view the best and most powerful combination class of the lot).
HD is an average of the two classes (e.g. a mage fighter would have 1d6, being between 1d4 and 1d8). Attacks as whichever class gives the best numbers. Initial weapon mastery as for best class, subsequent advancement by total character level (with extra fighter slots only available if the fighter level warrants it). Half XP bonus for prime requisite of each class.
These characters are genuinely multi-talented; they don't necessarily have the same restrictions as those using the paladin approach. So a cleric-thief is someone who joined the clerical establishment but already knew enough about being a thief not to need to seek out further training. A mage-fighter was probably a figher in an earlier career, and then somehow got an apprenticeship and just never stopped throwing his sword arm around. There will, of course, be some social stigma, but it isn't -required-.
In theory, a character can learn a second class at any time, but it won't necessarily be that much use to them. The first twelve levels might sound like free power, but then the rate of advancement of a mid to high level PC would be slowed for the gain of a paltry amount of power at each level to begin with. The price associated with learning a new class could be potentially immense, of course (so, why ought we accept you, a massively powerful thief, into our clergy?), possibly involving some kind of quest or payment.
Theoretically, either of these approaches could yield any combination; if it makes sense in the campaign, and the character has the stats to pull it off, why not? I can't see a mystic with a second class somehow, and I would strongly advise against something like a thief assassin (thug) (it's pointless), but other than that I've left that intentionally loose.
No one has played a triple or quadruple classed character. No one has ever asked to. I've therefore never playtested anything for that.