I have 10 fish and they have all come from the same local fish store that is about 10 minutes from my house. Prior to these last two I was not doing QT. As to acclimination I float the bag for 20 mins and then add a small amount of tank water every 5 mins until it is doubled. Thhat usually takes another 30 mins. Then I pour most of the water out of the bag and let the fish swim out.
Thanks,
Process you described is fine, i am guessing just bad luck and maybe already sick fish or shock.
did you check the salinity of the LFS and compare with yours?
most lfs keep salinity lower and that would explain sudden shock and death.
just my guess.