I can see two arguments:
1. Once you select a class-specific multiclass feat, you count as a member of that class for the purpose of meeting prerequisites for taking other feats. Since you cannot take a multiclass feat for your own class, that means you can't take a second class-specific multiclass feat for the same class.
2a. On the other hand, both Martial Power and Arcane Power have class-specific multiclass feats that have another class-specific multiclass feat and paragon multiclassing in the same class as prerequisites.
2b. Plus, the character builder says you can.
On balance, I favor option 2.
