My two female German Blue Rams will stay together 100% the time... when they are not fighting 
I caught them fighting on video
[video=youtube;IFKK71QD7UM]https://www.youtube.com/watch?v=IFKK71QD7UM[/video]
Every time it happens, they will be aggressive to each other for a day or so, then one will "lose" and always get "bullied" around for the next a few days. Then everything goes back to normal, "we are friends again"... until the next time. The winner switches...so not always the same one wins.

I caught them fighting on video
[video=youtube;IFKK71QD7UM]https://www.youtube.com/watch?v=IFKK71QD7UM[/video]
Every time it happens, they will be aggressive to each other for a day or so, then one will "lose" and always get "bullied" around for the next a few days. Then everything goes back to normal, "we are friends again"... until the next time. The winner switches...so not always the same one wins.