Bruins, a fight for the alpha female position might well be what is going on. As you said, terriers do have stronger tendencies along this line. But, I will offer another possible explanation for what is going on. Just as with humans, sometimes two dogs simply cannot stand each other! There appears to be no reasonable, rational explanation for it. They just don't like each other!! It might sound silly. But, I know it does happen.
We have two female Poodles that are like that. For no logical, apparent reason... they
loathe each other!! Both are great, loving dogs. They get along great with the rest of our pack... play with them... share toys with them... share food with them... everything's peaches and cream... except with each other!! My wife and I have witnessed each of them literally aggravate the crap out of the other... for no reason... until a fight ensues!! For that reason, we don't dare even go outside without taking them with us or separating them.
I don't know that this is what is going on with your two gals. It might be that its a combination of what you and I each said. As I said, I don't know. I just hope you have good luck at finding one of the girls a good home... maybe where she'll be the only dog... and have a huge back yard and plenty of room to romp, play, and work off that inherent Terrier energy!!
Happypipester, I'm glad to hear that your two girls are getting along so well... and, so quickly and easily!!! I hope the road that lays ahead is a smooth and enjoyable one!!!
Good luck to both of you gentlemen... and to your four-legged children!!!
Robert