Noise in cascode opamp
I think superposition might cause some confusion.  Here is a different way to think about it.  M2 can be thought of as a source follower.  This means M1 current will flow into the M2 source effectively shorting out ro1.  That's why there is no current division in the first term.  Again thinking of M2 as a source follower, its gain will be one.  Since M2 source is equal to M2 gate, M2's current is -vgn2/(rs2+ro1||ro2).  In this case the current is split between ro1 and ro2.  Only the part that returns through ro1 contributes to ion.  The ro2/(ro1+ro2) is the current divider that accounts for this.  I never see small signal fet models that include and rs term.  I think it was done here to justify the math.  If rs2 is very samll, multiplying the ro1||ro2 resistance by the current divider will give the -vgn2/ro1 result directly.

The confusion using superposition might cause is thinking setting the voltage source to zero means the dependent current source is 0 and can be replaced with and open circuit.  This is not the case.  when vgn2 is 0  the changing M1 drain current will change the source voltage at M2.  This will cause a change in vgt2 voltage the prevents the M1 current from flowing through its own ro.