There are good reasons I'm only flying one headsail in the image below:

1) The inner sail (under most circumstances) is likely to mess up the airflow and lift of the bigger sail. The "slot" is an area for added turbulence that can interfere with the lift of either or both sails. Turbulence = drag.

2) In my limited experience, I don't see much benefit from it unless it's continually tweaked. Maybe fine for racers but we're lounging under the bimini in this image.

3) It greatly complicates tacking, and is a PITA for a short-handed crew. (The wife and I would likely furl one sail prior to tacking, and that would be the outer one, as it's the one most likely to foul against the luff and stay of the inner sail. Furling a sail that's on a continuous line furler requires running off to blanket the sail with the others and reduce apparent wind to relieve pressure on it. Running off is not something we want to do if trying to sail to windward.)