The recording devices are two identical Zoom H2 Handy Recorders. They have a 90 degree angle mic in the face of the machine and a 120 degree angle mic in the rear. So - I have front-facing one before me to pick up (mainly) the guitar, and a rear-facing one in front of Al to pick up (mainly) the box. The 120 degree angle picks up both right and left hands of the Jefferies Anglo concertina.

I fed both recordings into Garageband software on my Mac and synched them. When they were synched, I topped and tailed them and then changed the pan and EQ levels. The video was shot with me on the left and Al on the right, so I panned the guitar Zoom signal about 20 degrees left and the concertina Zoom signal about 20 degrees right. I then adjusted the volume levels so that I got a decent balance and - hey presto!

I then hacked into Al's bank account and sucked out his pension - well, not really...