Well, you can roll it however you like. You can take levels as you see fit, no specific order or number of each classes.
A bit of background for you, originally we started rolling toons lvl 1, then roll for your next class, then roll for next, then roll for your next, at each level up. But, this proved to almost guarantee to gimp most toons because:
1. You had no clue what your next two classes would be when you allocated your starting stats, skills and feats;
2. You had no clue as to what your 3rd class would be when you got your 2nd class;
3. The probabilities of being pure (or 2 class) were extremely low (if not impossible);
4. Your class level splits were all based on rolls, and not at all a build strategy so this made 99.99% of toons gimped. We had a lot of complaints although this was also very fun in its own way
So, yes, you can multiclass but have to roll for the build. You can spread your classes out how you like considering you know your classes before you make your toon. I honestly suggest that everyone running TRY a pure before even attempting to roll multiclass.
Also, the whole point of this experiment is to have multiple toons, keep them alive, and see what sticks!