The circuit is a series of Isource and Z, where Z is the impedance due to RL and the MOS. In general, if Vi is the voltage across the Isource, will be:
(Vdd-Vss) = Vi + Z*Iss
If two MOS are driven alternatively into conduction, Z will be approx equal to RL so Vi = (Vdd-Vss) - RL*Iss
If, instead, they can, under some circustancies, go from both open to both closed passing through the linear region:
Both open: Vi = 0 since Z tent to infinite - This is the minimum possible current
Both closed Vi = (Vdd-Vss) -RL*Iss/2 since the twor RL are in parallel - This is the maximum possible current
Linear zone, depend from the impedance of each MOS channel