Three ways I can think of. (1) Angle the mirror so the camera actually sees the reflection of the second actor. This is done all the time to hide the camera. (2) Don't use a mirror, instead have a frame over a hole in the (possibly fake) wall and the second actor on the other side.
(3) Cover the mirror in chromakey green and use your favorite software to drop the other actor in in post. If prepped right this may even be quicker than option 1 or 2.

RJSchwarz
San Diego, CA

thanks! I actually though about the 3rd option but I wanted some other options. I guess i'll do that one!

thanks 🙂

This would take about a minute in an effects program like after effects. Just shoot the actor walking in on the left side of the shot and have the mirror on the right. Shoot this twice, one with the actor that is infront of the mirror walking in and the other with the actor that is reflected in the mirror walking in. Then in after effects or whatever you have cut the two clips and layer them so the left side shows the actor and the right side shows the 2nd actor in the mirror reflection.

"Imperfection equals Realism"

Henry's way is probably easier and will have the best end result.

RJSchwarz
San Diego, CA