How long have you had them? If one survived and one didn't, maybe it was just that fish's time to go. Sometimes fish just die and there's nothing you can do about it. If the water parameters are good, you acclimated them and they have no sign of disease or damage from other fish, then maybe it just died of natural causes.
Or it could have been the stress of the move itself. Was it hard to catch it? How long did it take to catch it? If it takes more than a few minutes for me I just let them stay another day to calm down before trying again. You could also hurt them while catching them. I've denied fish from certain LFS's after watching in horror as they squeezed them against the glass and rock trying to catch them.