Sheryl, consult with your vet about the benefiber. I'm sure it will be ok but you need to get the correct dose for you little ones. You don't want to give too much because it may back up the stool and cause an impacted colon. Just check with the vet, have your girls weighed and then you can come up with a good dose for the girls. But i think it would be worth a try.
Definitely worth asking your vet about, just to make sure there's no underlying issues (sensitive stomach etc) Have you tried changing her food?
Canned food always gives my cats diarrhea and certain dry foods, its been a case of trial and error to find one that suits them.
I have switched their food. We started with Science Diet I/D, then went to Wellness, now I'm trying Natural Balnce Limited Ingredients Diet, Green Pea & Chicken. Also tried raw, but they wouldn't touch it. Lilly did for two days, then quit. She had good poop with the raw.