Those figures aren't too bad, hopefully the water change may have brought the PH down as that has crept up!
If these are the only 2 of the 12 recent purchases then there could be down to them not being able to cope with all the moving around, especially if you got the from a LFS. They probably went from breeders tank, to shop tank, to your tank in a short time and all with different water parameters. To lose one of 6 would be normal for me from when I bought them at LFS so try not to over react at this point! If some moulted ok then maybe the shrimp that died weren't quite as healthy/strong? Buying from a specialist will be 1 less step/water and they were probably kept in water better suited to them.
Let us know if you still get any deaths though, after 3-4 weeks the shrimp should have moulted once and be well adjusted to their new water parameters so you shouldn't see any problems from then on???
Simon