I've done that in the past with a piece of glass on the edge of frame, angled to reflect the background, as in this frame where I put the glass on the left side of the lens -- I could have gotten a second reflection with some glass on the right side as well.
I'm trying to wrap my head around the optical physics of how you resulted with that look, David. Is the glass much closer to the lens than the actors, so physically out of focus, but its reflection optically reads as the same distance the actors are? Not sure if I even worded that question right.
Sure, the reflection is the same focus distance as what it is reflecting just as with a mirror reflection -- if you hold a hand mirror right up to your lens and reflect something 20 feet away in it, you wouldn't be focusing to the distance between the lens and the mirror if you wanted the reflection to be in focus.
The advantage of using angled glass on each side of the lens, open in the center, to create the effect of the reference above, is that unlike mirrors, you get a more subtle overlap and ghosting effect, the reflections on the sides are semi-transparent.
Just went to a department store and bought some picture frames and took the glass out. In my case, I got something large like 9"x14" but if you were going to build a black box with a piece of glass at a 45 degree-ish angle on the right and left third, you can probably use smaller pieces of glass depending on the size of the lens. Generally these shots work better on longer lenses with a flatter perspective.
The advantage of having each piece of glass on a c-stand is just that as you change focal length, you might want to adjust the size of the clear gap in the middle. But the disadvantage of not building a box that can attach to the front of the lens is that when you dolly or pan, you move through the glass, it doesn't stay aligned. Which sometimes is cool, to slide back and forth through the reflections.