Not 100% sure and as far as I know no gameplay footage from the full game has shown multiclassing in action, but my impression is they are trying to separate to some extent the levels of each class to...so if you have 4 levels of barbarian and want to take a level of Wizard at level 5 that level will not exactly be the 1st level of Wizard so instead of receiving the level 1 stuff you receive what a wizard would receive at level 5 without having the stuff of levels 1-4...though I am assuming there is some exception there for something like subclass and maybe level 1 cantrips. So in a way it would be like having level 5 wizard benefits while only having one level of wizard.
I highly doubt it will work like that or they've thrown any semblance of balance out of the window...
It is pure speculation until the game is released anyway. There simply isn't enough information about multiclassing, except that they've removed the ability restrictions (which is a decision I think will prove to be bad).
