like all blennies, bicolours are terrotorial. When you think about their feeding behaviour (algae, etc), it is not surprising that they exhibit territorial behaviour. If you could sex them, I would have a pair, and even though the lank is large, there is the possibility of obtaining 2 non-compatable individuals that will cause stess to each other in the long term. I would stick to 1, but that is just my oppinion