Fighting gerbils?

I have two girls from different litters but approximately the same age. Only a few weeks difference at most. They've been together for 3 almost 4 months now and there have been no problems. They slept together in a little ball, even ran on the wheel together.

Today, the red one started chasing the smaller black one, and the black one squeaked when cornered. They play chase occasionally but not this rough or fast. It took me about 10 minutes of rough chasing to figure out I needed to separate them. I pulled the red one out for 20 minutes or so in a tote-size carrier, trying to give some cool down time. Reintroduced them and they went right back at it. They live in a sizable terrarium with plenty of food and water and shavings, so no resources were low.

I tried two or three more times to give a "time out" but finally just decided to separate them for the night. I put the troublemaker in her own emptied out 5gal aquarium (used to have a fish) for now. I was wondering what the odds are that I can put them back together in a day or so. This is a totally new thing, so I'm hoping it's a phase. Any suggestions or tips would be great.