How strange...for one to go missing like that...two is just wierd !
Here's a thought...sorry if its a bit gory...the little conical snails that live in the gravel (I had a plague of them when I kept Cichlids) could the fish have died and they pulled the bodies under the gravel !I had an Angelfish die suddenly in the night once and in the morning it was covered in them !
On a lighter note, I know you are very attached to your fish but you can look for some new little gems now.
Edit...Not sure what I did wrong for this reply to appear in same box, never happened before
I found the head of the yellow male, the snails eat the dead bodies. My friend came over with another yellow male and a half black male with a red tail. We will see if they last. I am so annoyed that my waters fine and the fish are still dying, this is what happened when I first got guppies and vowed never to have them again but they are so pretty.
It's true, but you never know what the babies will turn out like and they are both obviously pregnant. They are both gray, one just solid gray duckling and the other has some promising markings on the tail and dorsal fin, but it's patchy so not pretty either. 
There were only three females in the store so there wasn't a choice anyway and they looked healthy.
