really??are they can't keep together?? but they just swim together and nothing happen.....i hope they can live together, because i haven't enough taank to keep many kind of the piranha, and i found one of the geryi in hk about 9", it cost $1800 hkd, i also want to buy it because he always want to bit my finger between the glass , but i haven't another tank to keep that
Try to make a habit of walking by them often. When you feed them... make your presence known.... and watch them for awhile. The more they recognize that you are as much as a part of the tank as they are, they will become less frightened when people are near. Get them used to light as well by keeping it on as much as the light allows.
I'm not sure about taking away all of their hiding places... I don't know if it would over stress the fish... but you should be able to get them comfortable by just letting them know you are there often. It will take awhile.