First, I would recommend installing new dividers so they can no longer see each other. You should be able to pick up closely thatched dividers that should do the job – hardware stores, aquarium, fish stores, pet stores may stock suitable dividers.
Second, I would baffle the filters if not already done so. If there is a strong current this may be partially to blame for torn fins.
Torn fins could be indicative of stressed swimming/flaring or tail-biting, whatever the cause we can assist healing with AQ salt and clean water. If your situation allows for 1g hospital tanks do so, this means it will be easier to dose, water change and monitor progress. If you have a 1g at your disposal place one betta into it with fresh water treated with conditioner – I recommend Stress Coat+, it has a useful aloe vera additive that assists healing torn fins and providing a slime coat. You can dose the tank with 1tsp of AQ salt. After 24 hours, you will want to undertake a water change of 100%, then redose AQ salt with equivalent used previously. Repeat this each day. You will want to continue with this treatment for 7 days until you see improvement, if after 7 days you only see slight improvement give him a break from medication (consistent use of medication will develop an internal immunity to its benefits), after a few days without treatment, restart. You will want to do this for all Bettas.
You don’t necessarily have to have a 1g tank, you could utilise a 1g jug or mug etc. The only issue when doing this is attempting to keep a stable and comfortable temperature for them, I personally use a betta heater for my hospital tank. Does the job but not ideal for perma use.