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.
Zdravko Veselin Gaeta
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