If they had bad teeth why would the hairball remedy affect them and not "regular" food?
For 2 months of throwing up at a rate of every 2nd day, there maybe something else that is making them puke,
and being your not having a diarhea problem to go with the puke, the other medical possibilities should be checked out too.
If the Vet checked the teeth thouroghly, at 14 years she would have some tartar build up,
My Vet also sent my cat away with a good check-up, a week later she went in to another Vet for a second opinion,
and not only did she have 3 cavities but also was hyperthyroid, don'yt know how that all got past the first Vet, but I still though something was wrong.
she had the worst problem with puking, whole food chunks, frequently, weight loss, it was so bad she was taking the steps one leg at a time because of the muscle loss from thyroid overproduction,
the red swelling in her throat was treated with antibiotics,
then the other cat that was puking was taken in and she also had redness and swelling in her throat, more antibiotics,
and neither of them had a diarhea problem, but did stop puking completely.
These 2 were also eating from the same plate when I feed them the canned food, and I suspect that they shared the bacteria this way.
My other cat was switched to Chicken soup for the Cat Lovers soul, and she had to get her old food mixed in 50/50 to cure her runs, she never had a puking problem.
They all 3 went in for a dental cleaning after that because the tartar and gingivitis could be part of the bacteria problem.
Switching food slowly may help, but if the 13 and 14 year old had never had a dental cleaning you could check into that too.
I'm going to get my cats teeth cleaned every other year minimally from now on, just hope I can find a Vet that does it without anesteshia(?sp), as it can be hard on the kidneys.
Good luck, I hope you find it is as simple as the food, I wouldn't want anyone's cat to be as sick as my oldest was when she got a clean bill of health from a Vet, and that's why I think you may want to get more diagnoisis.