Hi Treybabe, Jo!
I think 2 females together will fight/argue more..........(not that it means you can change that now, obviously)
I have just always heard that, and my vet told me the same thing.

I think that when the younger one is a little older and more mellow, it will help.
I have 2 males, one is 8 yrs., and the other is just over a year. I have noticed that most of the time when they 'argue/fight' it is because the youngen' wants to play, play, play, and won't stop until he gets hissed at &/or swatted.
He has seemed to mellow (a bit) in the last couple months, but still has a way to go!

In the mean time, maybe keeping them separated at night, and providing each ot them with their own litter-box and food dish might help.