I have a female Betta aquarium and it is 215L heavily planted with lots of bogwood etc, and I have in the past had to take a female out "for a rest" as they can have a go at each other and she was showing signs of stress. You may find your 5 gallon is too small for the girls. I was also advised to have as many as possible to prevent any one Betta being "bullied" by the others. it will all depend on the age, temperament etc of the two, you may be lucky and find they are OK. One thing is to re-arrange the aquarium BEFORE putting the new girl in there. This then breaks down any favourite areas of the existing female i.e. she does not defend her territory. Also as well as plants have plenty of hiding places that the girls can get away from each other if need be. I currently have three new girls in Quarantine and they are only 3-4 months old, they can still flare at each other from time to time, but generally hang out happily together. There are plenty of nooks and crannies to hide in if they need to take a break. I take it the CT has been Quarantined? Keep us posted ...