Allie, I'm very sorry for your loss. It takes a long tme to get over a pet dying, especially if you put it down.
I put my dog down in 2003 and I still dive under the table to greet him when I get home to my parents' house only to realize that he's gone. Sometimes, when I'm alone I call him, forgetting that he won't react.
Hang in there. Oh and of course your other dog knows what's going on. They always do.