Not trying to sound like a d*ck or anything, but stanzzzz is right.
That tank is WAY to small for dovii, even as young fish they need lots of space to claim.
If you can rehome them or give them back to the store that would be best. There are lots of fish that would work to make a beautiful display in a tank that size and not leave you with a massacre.
As for what to do in the mean time... I have no clue. Do you have a large rubbermaid tub you could set up as a temporary tank? Normally I'd suggest a temporary divider but in a tank that small it's cruel to divide it and force the fish into an even smaller space..
But then I guess its also cruel to let him kill her..
Which is the lesser of two evils? I honestly don't know.
Either way good luck. But please, get these fish into a better (bigger) home asap.