Simple NMOS transistor circuit output impedance
So I spent some time researching the problem, and I came up with a possible solution:

First we draw the small signal equivalent for the circuit and add a test voltage \$v_{\text{d}}\$:



I have made the assumption that \$r_{\text{ds1}}\$ = \$r_{\text{ds1}}\$ = \$r_{\text{ds}}\$ because the transistors are identical.

We find expressions for \$i_{\text{d}}\$ in the top part:



And for \$i_{\text{d}}\$ in the lower part:



We seperate out \$v_{\text{s}}\$



And insert it into the first expression for \$i_{\text{d}}\$:



We then solve for \$R_{\text{s}}\$:



Which is the output impedance.

If anyone could verify if the answer above is correct or incorrect, that would be great.