I had a similar situation and essentially extended the return to the top of the stairwell on the second floor. Now, the difference is about 2-degrees. Two things, I usually leave doors open, so it is a clear shot for air to circulate and, I usually leave the fan on continuous. This helps keep the air from stratifying. I basically close the return downstairs and leave it closed all the time now. If you close your bedroom doors, a single central return won't do much, regardless of where it is to keep things evened out..