I have another question about these bettas.. How long will it take for his fins to grow back?
well it all depends on the damage inflicted and general health of your Bettas. If we could get some pictures we might be able to give you a closer estimate.
to stimulate fin growth though...
well first to prevent infection get some medications to help prevent infections, there are running debates on here, so I am pretty sure you'll be able to figure out which is best for yourself.
make sure that your water's clean and do regular 25% water changes. I'd personally use Melafix since it's always done me well.
an ideal water temperature would bo 80 or more degrees F or 27-28 celsius.
to deal with stress and give your fish much needed electrolytes, add some aquarium salt. (please read drections)
If at all possible make sure that the water in the tanks does not mix to prevent one Betta from possibly infecting another.
I'd say that with regular treatment and some good old TLC you'll see a huge difference in two weeks.
(please feel free to correct me if I have made a mistake here. I am no expert in the care of Tropical fish.)