Peter:
Click on both links and they will play @ the same time.
With NR's videos you have to cue the first video @ 4:10 - then click on both to play.
I'm going to sleep on it and see if I can come up with another combo that works together.
I used to do something like this in the 70's/80's when I had a dub cassette deck with synchro, but I was sourcing original music from my guitars and keyboards.
My first recording of this type involved different revolving/inter-lapping time signatures used for self hypnosis/meditation.
All of my musical cassette recordings (including these experimental ones) were stolen from the back of my car when I was moving house in 1991.
DeKay