Why isn't it as easy as this to prove:

Dreamer "A" is given a word by experimenter right before bed. Dreamer "A" is to make contact in the dream world with Dreamer "B". In the dream, Dreamer "A" tells Dreamer "B" the word. Upon waking, Dreamer "B" tells the secret word to the experimenter. If the word is correct, dream contact occurred. To make it cheatproof, the experimenter would have to supervise one of the dreamers during the night to foreclose the possibility of communication.

The fact that it is this easy and yet has never been done, as far as I know, makes me skeptical.