Imo you could get away with doing a few sets of curls on just one of the days and see benefits. However, you could also do a few sets on both days at different rep ranges. For exampel upper body day 1 do 3 sets of 8-10 db curls, upper body 2 do 3 sets of 12-15.
I've done both and they both seemed to work.
"The fastest way to get strong muscles is also the fastest way to make them bigger. We know what works best to make them strong in the shortest time possible. If you want to get big, do the thing that makes them strong." - Marc Rippetoe
Zdravko Veselin Gaeta
I am not a powerlifter nor am I a bodybuilder. I just want to be big and strong.