If you are working back and bi's on different days than you are basically hitting your bi's twice a week. I work push movements together and pull movements on the same day and hit each body part once per week.
Mon-Back and bi's
wed-legs and abs
Fri-chest, tri's and shoulders
Like Joker said everyone is different so mix up your routine and try something new. If you are prone to overtraining and plan to add another arm day I would start out light. Just my $.02.