It depends a bit on where your roots are for how you interpret harmonica:
http://en.wikipedia.org/wiki/Harmonika

Anyhow, for the best strategy it seems obvious that the person in front must go slow enough such that the person behind is able to rest and do a pull at a speed faster than the solo speed of the stronger rider. Doing a quick rotation of 30-60 seconds also seems to be preferable in order to be able to consistently pull at a significant level above FTP.

I just don't see a reason why for just 2 people the effort up front needs to be the same for both riders. When there is a large pace line, you want the people that are resting in the middle to be able to rest at a constant pace while they wait their turn. But when there are only 2 people the two riders are always switching between high and low power anyway and they can react much faster than when there is a large pace line.

I might be over-thinking this and maybe I just need to try it out. The other day I was trying to go around a 30 minute loop as fast as possible with a friend and I was 2 minutes faster than my friend. So we were wondering if we could improve my best time by staying together or if the difference between us two is too big for a pace line to work.