You can conceal your identity without drawing attention to yourself. Wrap around sunglasses, wear your hat the correct way and low. Don’t wear a bright white Tee under your jacket. Those things alone would make it harder for us to distinguish them. Not completely, but harder.
So far we don’t know the identity of either of them. They weren’t drawing attention to themselves. They were just part of the crowd and no one has come forward to say they say one of them place the bomb down. They just melted away undetected.