Well... if you are really artistically inclined and have the time, money and effort, you could go about making an actual mask...
1. Get your "before" person (the one who is what the mask is representing.) Make a plaster mold of their whole head. Then make a cast of it as well. Pour liquid latex into the mold and place the cast in the mold, thereby properly shaping the latex the same as the cast.
2. After the latex dries completely, remove it from the cast and mold. Powder it all over (baby powder works) or else it will stick to itself and ruin it. What you effectively should now have is a thin rubber version of your actor's head. Paint it according to your actor's flesh color, and apply fake hair in the color and style of your actor. This can be either punched in individually (Time consuming) or buy crepe hair from a halloween online store and glue it on the head (cheap and easy
3. Now, since it is a mask, you can put it on your "after" actor's head. Since it will be an obvious mask on camera, make the cut right as the actor reaches to rip it off. He can probably literally rip it off easily since it is very thin rubber. If not, he can just pull the mask off quickly.
An alternative, if your actors have very similar hair, then you can just make a cast, mold, and mask of the face alone. This makes it a hell of a lot easier and faster. Then you use adhesive to glue it to the other actor's face. Depending on your skills, you may actually be able to make it realistic enough to wear for longer than cutting just as it's ripped off. Also, other, more flexible materials may be available to you for use. Instead of latex, foam works (but it must be baked) and prosthetic gelatin (which may just be the best stuff out there, aside from the fact that hot lights melt it. Oops.) With all of these materials, it will actually move with your actor's face, making it more realistic. Just look around. Mask-making is a fun thing to get into. Try it out.
P.S. I realize that this effect probably could easily (and quickly) be done in After Effects or a 3D app. I just did this because:
1. I don't know much about 3D or AE.
2. I am a big fan of doing live effects whenever possible.
3. I also like mask-making and realize that there are others like me out there too.