What else do you have in the tank? Anything with a mouth big enough could have eaten them.
Do you have lots or ornaments and plants in the tank? They may simply be out of sight. They much prefer to be in large groups and having only 2 may make them quite shy.
I've just got 5 blue neon fish, and 2 small African frogs which are know where near big enough to eat them at all. The shrimp are the biggest things in there.
There's some grass in there but I've searched through it and still can't find any of them?