Ideal operational amplifier resistor question
Yes - you can (you must) consider both resistors in series if you are applying the superposition rule for calculating the closed-loop gain. 

Based on the assumption that the inverting opamp input has a potential Vn=0 , we have a simple voltage divider consisting of the series R1 and R2:


Vn1=Vin[R2/(R1+R2)] setting Vout=0,
Vn2=Vout[R1/(R1+R2)]  setting Vin=0.
Vn=Vn1+Vn2=0  >>>>  Vout/Vin=-R2/R1.


Comment: Note that the voltage divider rule was derived/formulated using Ohms law and the assumption that the current through R1 and R2 is exactly the same.