Even though a unit can only use the Wave Serpent to which it is assigned, they can be deployed in three (3) ways, I think:
1) Unit is inside the Serpent.
2) Deploy the unit close to its Wave Serpent
3) Put the Wave Serpent wherever you want in your zone, not necessarily beside the unit. You still deploy both unit and the tank at the same time.
Option 3 is cool if you want to split your opponent's fire between the "container" and it "contents".