Should you be devoting a single day towards training Biceps or, Triceps?

The information is widespread that biceps and triceps are a very small muscle, so they require much less amount of training than bigger muscles, like Back and Legs.

And, there are other sent of advisers, who propose that, they do not require training at all, since they are already being trained on Back and Chest days.

In my opinion, one should train them separately in his initial 1-2 years on separate days for full muscle growth optimization. 

And, whenever, one feels that he has hit a plateau in terms of arms size development, he should change his routine towards a more robust biceps + Triceps training approach wherein you train them on the same day.

And, again, if you have hit a plateau, change towards a plan where you train them less frequently but with more intensity along with bigger muscles like back along with biceps and chest along with triceps or, vice versa.

There is no fixed criterion on how you'll merge your workouts, i personally felt that training muscles like these require not a lot of weight but a hell lot of intensity and variety. Super setting is one such way which I found really helpful and, always train these muscles to failure for best results.