current value of two serial transistors
Don't know how to write equations here, so let me follow the traditional way. In the second figure, the mosfet is in linear region. So we can write its current Ids1 as:
 Okay now in the first figure, currents through the series mosfets are same and its equal to Ids2. Both the mosfets are in linear region here. So we can find I1 and I2 separately and equate both to reach at a final expression for Ids2. The final expressions for Ids1 and Ids2 are found to be the same. Hence Ids1 = Ids2