Maybe they're fighting? Usually you only want one tang per tank because they don't like their own kind. It could happen that one may harrass the other to death, I know that sounds bad. I had two talbots damsels and didn't know they are loners, one did this to the other a lot, but I could never catch them to remove them, so one ended up dying.
I just today had to remove a black damsel because the lfs
told me they could be together, but upon reading about them and watching one harrass the other, I found that to be bad information.