Why can't they meet each other?

My vet had said to keep Melly seperated from our roommates cat, an approximately one year old spayed devon rex until she was spayed & had her second distemper shot & rabies shot. Melly was also sniffling then, so we definately wanted to keep them apart. Now Melly was given a clean bill of health, but still hasn't been spayed or had her shots. What's so bad about them meeting each other now? Can they at least sniff each other from under the door? Right now we have a towel beneath the door of the room Melly's in. Would her second shots kill off anything if she had it? Or are they just vaccines to prevent it?