When my arms were lagging I dropped the weight on isolation exercises and really focused on form and the contraction. I'm talking like 20-25 db curls. Just an idea
Edit: Just a reference video I liked.
Kai Greene - I'll Never be A Weightlifter. - YouTube
You're not gonna get 20" arms curling 25lb dumbells.
The only way to force a muscle to grow is to make it stronger, higher reps with lots of volume definatly helps the muscle to grow too because of the metabolic fatigue, but you cant just keep pumping the 25s forever. you'll never see a big bodybuilder whos weak.
To answer ops quesiton, yes twice a week is the best for arm growth, have one very heavy day and one with pump day (12-15 reps).