I'm assuming you're referring to the mirror image......

At times acting like a true mirror and, at other times, showing the same people playing different instruments?

Ok, if I were to try and recreate it, this is what I'd do - and keep in mind I haven't really studied the clip:

The key to this effect is that the mirror has two sections, with a large frame around each section, separating them.. The guy's mirror is left alone, the girl's mirror becomes "target 1"..

The reflection of guy playing acoustic guitar always seems to be just a regular mirror image of him.. So, while filming the main scene of them playing, you just leave the reflected image of him in HIS mirror "as is" in the clip you shoot.. That's the NORMAL reflection and adds realism to the video, which makes the girls reflection stand out more..

You create another layer for the girl's mirror and the reflection clips of them playing different instruments you will shoot.. Its like adding additional "tracks" to music pieces.. In a studio, all instruments are on their own "track" so you have complete control of each instrument in order to mix it.. in this case, you have complete control of all movie clips and can edit them any way you want.

So the girl's instruments change in the mirror reflection: sometimes they are showing them playing different instruments, sometimes the same instruments.. So, you have at least two film clips of the girls playing different instruments, reflected off the mirror.. so, you hafta film - separately - clips of the girls playing each set of instruments in the reflection off the mirror..

You film them playing normally in the room, pan the camera to the right to show the mirror, and at the times the reflection is correct, you just pan back toward the left showing them in the room so that the mirror is not visible.. for the times they play different instruments, you splice that mirror footage of them playing different instruments into your main clip on a layer above the main clip.. You can crop the mirror clips and skew them to fit the mirror angles, if necessary.. You splice in that mirrored footage when the camera is panned far left and you don't see the mirror.. then you pan right and BOOM - there's the different instruments in the reflection.. They only change the mirror reflections when the camera is panned far left and you don't see the mirror..

There is some chicanery afoot in the clip.. possibly multiple times, but skip to the 37 second mark and watch the guy's guitar sliver in the girl's mirror.. its there, then it just disappears cuz somebody did a poor editing job.. also a something light colored in the same area gets lighter at the same time the guitar sliver disappears, so THAt was the second they spliced the footage of the girls playing different instruments into the girl's mirror.. it pans all the way right and - yes - they are playing different instruments.. I would bet if I go through the clip, I could find all the spots where they spliced in the different instrument clips..

Ok, I thought of THREE ways to do that effect.. The one I rambled on about was the easiest to explain.. What they probably did would be much more difficult to explain, using multiple movie clips, a camera in your project that pans the multiple clips etc... Its an interesting effect, which really isn't difficult to do, but kinda difficult to explain..

---

Friendship is like peeing your pants.. Everybody can see it, but only you can feel the warmth..