Providing cover and places where they can dig holes under stuff might help them too.
When I first saw my 4 damsels carrying rocks around in their mouths I was pretty amazed. Turns out they all dug their own home in a corner, under a piece of liverock, etc. This kept them happy.
Conversely, when I moved them all to another tank, they all werre scared for a little bit and then became "friends" and now they sort of "school" together. (No kidding).
I agree, take aggressive one back to LFS
if he won't stop blasting the other one even after you provide them with cover, and give the ragged guy some cover and wait until he heals up.