Cable Double Biceps. A good exercise, in that it's very difficult to cheat. Unfortunately, most will cheat reps on biceps work, whereas it's tough to have "bad" form when performing Cable Double Biceps.
On a side note, Cable Double Biceps is also a great exercise for teaching would-be bodybuilders how to perform the Double Biceps pose correctly i.e. elbows slightly higher than shoulder.
For ideas on how best to incorporate this exercise into your training regime, please consider the purchase of the acclaimed Gym Workouts: Maps to Success or download (for free!) our workout app GP Shuffle. For the best arm workout ever, you should also consider GLOBE GRIPZ™.