Maryn lays it out fairly well. You want to establish the characters in each location, perhaps how/where/why the spyhole is located where it is. The character puts their eye up to it - CUT TO
The other side, and everything that happens, that the watcher sees.
Intercutting as Maryn states is useful. Much as a phone conversation is done - or a conversation that takes place between one person in a vehicle, the other outside of the vehicle. You don't need a full scene slug for what's happening in each location.
If the watcher is alone - they are essentially doing nothing but watching - you don't even need to intercut.
BOB'S LIVING ROOM
Bob puts his eye to the crack in the wall. He watches closely as
MARY'S BEDROOM
Mary walks slowly over to the closet, removes the hat box from the top shelf and takes the money bag out. She goes through a lot more bizness... Until the necessary information is revealed CUT TO
BOB'S LIVING ROOM
Bob slides the picture back into place, and picks up his drink as if nothing had happened.
Now, if Bob is DOING something, in concert with or in oppostion to what he is seeing in the other room - then inter-cutting might be necessary. This can be done with SCENE SLUGS or CHARACTER SLUGS.
You've already established the essential location - Bob's house. You have established Mary is in the bedroom, while Bob is in the living room.
INT. LIVING ROOM
Bob removes the picture and looks through the spy hole.
INT. BEDROOM
Mary closes the door and puts her ear to it, listening for a moment.
INTERCUT BETWEEN LIVING ROOM AND BEDROOM.
MARY - Moves to the closet and opens the door.
BOB - Lifts the sofa cushion, and removes the gun.
MARY - Glances around - reaches for the hat box on the top shelf.
BOB - Watching closesly, even as he screws the silencer onto the gun.
MARY - Pulls the hatbox down, opens it, and removes the leather pouch. She opens the pouch to reveal the cash.
BOB - Silently chambers a round in the gun.
MARY - Places the money BACK in the box. Puts the box back on the shelf.
BOB - Places the gun with the silencer, behind the sofa. Moves to the chair and picks up his drink as if nothing had happened.
INT. BEDROOM - Mary stops to adjust her makeup in the vanity mirror, then opens the door to return to the living room.
It really depends on how long the scene is, what sort of action is being viewed, and whether or not the 'spy' is 'active'.