This effect is intended to be done in the context of a supernatural or fourth dimension story. Both sides of the slates are shown and then are secured together face-to-face by rubber bands. The previously blank faces which were secured away from the outside world, when opened for viewing, will boldly reveal the name of a previously freely chosen card!
All this happens while the slates are in the hands of the spectator and it is the spectator that opens and displays the slates!