you need to use mirrors not scrims. Actually not mirrors but glass or other plastic, that will appear reflective if there is no light behind. you need to create a black "box". For best effect the box needs to be pitch black inside. The Glass "mirror" is mounted at a 45 degree angle to the audience. place one person behind the glass and one person facing the 45 degree serface of the glass from the side. Depending on which actor is lit depends on which actor the audience 'sees'. There
Cross fading between the two can create a morph effect. obviously the effect is better if care is taken to make the actors appear to be in the same place.

If the person needs to emerge from the box then start with the person on the side seen first. When the other appears you can swing the glass out of the way with a little misdirection and have him walk out. All effect lighting needs to be accomplished behind or to the side of the glass not through the glass. Ohterwise the reflective light cast from the glass will spoil the effect.

Now having two people become one. Play with this and you may be able to get close to what you want. Good luck