When you calculate the gain and the input impedance of the shown circuit - in particular for unsymmetric operation with Vin1=0 (or Vin2=0) - you can treat the whole device as a common-drain-common gate combination.

Hence, it is not wrong to treat the system as a two-stage amplifier.

When you calculate the gain and the input impedance of the shown circuit - in particular for unsymmetric operation with Vin1=0 (or Vin2=0) - you can treat the whole device as a common-drain-common gate combination.

Hence, it is not wrong to say that the system acts as a two-stage amplifier.