I hated feeling railroaded into playing humans in the earlier BGs, due to dual classing. I certainly hope they don't' do the same in BG 3.
From what I understand there is no difference between dual- and multiclassing in 5e, and it doesn't seem to be restricted to race or class combination.
Multiclassing works pretty much the same as dual-classing did in BG2 in the sense that you need to meet a main stat requirement, in this case a min ability score of 13, but the system is way more flexible. Multiclassing in 5e is described as (paraphrasing): On every level up you choose which class you will gain a level in. The combined level of all you classes defines your current character level. So it's pretty much a very flexible dual-class, where you pick how and when you want to gain levels in your repecttive classes. In BG2 you would level up, as a multiclasses, simultaneously but slower and, as a dual class, you were forced to choose when to dual and then gimp yourself for a long time before you regained the level in your new class.
We have the opportunity to play around with multiple classes at once in the EA version, as the game lets you choose to gain basic understanding of an off-class as an option every 4th level. I have no idea if this is Larians interpretation of multiclassing or if this as entirely different option covered by some other part of the rulebook.
If anything needs to be corrected, or can be expanded, feel free to do so if you're more experienced or knowledgeable.
