What is the advantage of concatenating two nMOSFETs M1,M2 with Source of M1 connected to Gate of M2?
It raises the gain of the overall circuit as often one MOSFET can't increase the gain dramatically. Also taking a quick look at your drawing did you forget the source resistors? It'll separate the FETs from ground and alter your resistances. For example the vgs will be nonzero so the dependent current sources will have to be taken into consideration.

Google common source with source degeneration for worked out examples to find the gain.

EDIT: when I say the current source needs to be taken into consideration I mean the equation for output resistance will in some way include the transconductance.