Very odd, especially as they die whether in quarantine or the main tank. Because of this, I doubt stress from isolation is the cause. It is good to see someone actually quarantining their new purchases.
If your main tank is only 2ft, why do you have 2 bala sharks? These fish prefer to be in a group of 5 or more and a 4ft is minimum for a few juveniles. I have a friend whose bala outgrew a 6ft tank!
How are you testing the water? Liquid tests are best and ammonia and nitrites should be 0 and nitrates below 25 (although this may not be possible if your tap water has higher nitrates),