We have pairs of females in 24 wide by 18 deep and they do really well. As Hypancistrus said they can be territorial, we have one female that will not accept another in her tank, but will accept a male. We've found there are less problems if they are raised from youngsters together.
If they are older, you could try introducing them and just be prepared to seperate if necessary, we have successfully introduced older females, but have always had a back up plan just in case.