If you want to do this with focus, the background has to be deeper than the distance from the camera to the glass and back to the actors, so there will be a difference in the focus for a pull, which can be helped by shooting at a wider f-stop for shallower focus. Of course there is the exposure balance issue, making sure the reflection is bright enough. Hopefully this is a night scene because otherwise a bright exterior being reflected in the glass will overpower the interior view. If it is a day scene, you'll need something to create a lot of dark area behind the actors, dark trees, a dark truck, etc.
You might try all of this with a pola effect as well. I was watching the movie "Murphy's Romance" and there's an interesting shot where you see the reflection of a bit crossing a daytime street in a glass window with a Help Wanted sign, and then they rotated the pola to reduce the reflection so you could see the boy enter the store and apply for a job. I tried this trick once, but it requires a very specific angle to the glass for maximum effect from the pola.
if this is a night scene, the actors could step into a small spotlight at the moment you rack to their reflections so that they weren't reflected earlier when you wanted to see clearly into the store. Or have the light dim up as well. "Munich" did something like this, for a scene in Paris at night of Avner looking into a kitchen store.