Josh Sawyer just sent out an update explaining multiclassing. It must needs be said that the system is a work in progress and they're soliciting feedback. Basically, they're considering a choice between AD&D style (as in, you chose your classes at birth and that's that), or d20 style (as in, when you level you can take a level of another class you are eligible in at will, and then mix and match levels in your two classes too taste. The difference in PoE2 is that you can only have two classes).
They think the latter system comes with some additional complexity and they're not sure they like it. The reason being they balance it like this:
Every level up a class gains 3 "resource" points. These determine your power level in that class. 0-2 points = power level 0, 3 = 1, 8 = 2, 14 = 3, 20 = 4, +6 = next level forever more (or at least in the chart in the email, which went to level 10). At level up you also gain a phantom point that isn't counted unless you multiclass. So a level 1 fighter has 3 "discipline" points. And a phantom bpoint. At level 2 if he takes wizard, the phantom point becomes arcane, and he gains +3 arcane for taking a wizard level, giving him arcane +4. But the level also includes a +1 to his discipline score.
The cost is you lag behind in power level, so that a 5/5 fighter/wizard has 4/4 discipline/arcane level while a level 10 character would be at 5. Presumably the AD&D system just relies on slower XP advancement (with xp's split between classes).