Well its always important to remember that with a group of female bettas of this strain they are still bettas and are still technically 'fighting fish'. They are aggressive, and while they will tolerate each other in the right conditions, you are still always going to see some kind of aggression or nipping, especially when you move or change anything.
Also, bettas are very unpredictable. Even in perfect conditions you can have a successful sorority going for months...and then one day have a disaster because on girl suddenly didn't like this other girl for some reason. Sororities are fun, and if done right you usually won't have any serious problems, but there is always that risk.
...Okay, so you have two females in a 1.5 gallon tank together? Thats just a recipe for disaster here, you need to remove one of the girls ASAP. Two girls together, especially in such a small space, is going to end with one or both of them severely injured or dead. There is a reason its best to keep females in larger groups, and there is really no exception to this rule with this species.
Go ahead and do another water-only 50% partial change on your 10 gallon to remove some of the salt in there, rearrange the tank(and remove all your girls if you can)and add the nipped girl you removed back in. Do a couple more water only 50% changes over the nest few days to remove s much of the salt you already added as you can and then just leave things be.....there is no use in treating healthy girls in a sorority with a few nips with salt. Stress coat is fine to add if you really want to add something, but clean, warm water is what is going to help heal the nips the best.
And yes, if you get another 10 gallon tank, provided whatever you have your current 10 on can handle the weight if you're setting them on the same thing, you can put them right next to each other.