I guess it´s the old "problem" with tattoos, sooner or later you feel something missing if a tat is only on one side
(though this is usually "just" a feeling, and not something truly unbalanced, so the mirroring picture is not a necessity) But usually when you´re over the threshold of the second one, there will come more than that..... Could be different with band-tattoos, though, since they have another purpose than "normal" tattoos. Of course they are all a kind of symbol, but I have the impression there is a different motivation behind getting a band-name/logo and a "normal" picture. But you tell me your view, cuz I am not sure about that.