In that small of a tank I find it hard to believe you can't find a fish. lol have you checked the back compartments, (you have a cube right?) maybe she jumped?
Anything in there that could have eaten her up by chance? Usually I can find my scooter with a little bit of work, he leaves his eyes out of the sand.
I know youre stressing now, but just for future reference i would think about upgrading your tank. 2 fish is ALOT of fish for an 8g tank. I have my scooter in a 29g tank which seems to be perfect for him.
I hope your fish comes out.