Years ago when Sylvan Wells and John Mays described the Bourgeois double tenon neck joint I started using it based on their tutorials which involved a full mortise on the top for the fretboard tenon and a separate mortise block on the underside of the top butted against the headblock.
I experienced moderate movement at initial string up, more than a glued extension, or full dovetail but compensated with a slightly higher neck angle. What was disconcerting was that at 6-12 months there had been more movement sometimes necessitating a slight tweak in the neck angle. After that they remained stable.
I tried reducing the size of the mortise so more top was glued to the headblock with some help but later found that Dana had gone to a integral one piece headblock, mortise extension and left a slight gap between the end of the extension and the top brace. As he glued his tops first he would then wedge in a piece of wood in the gap to make a tight fit of the headblock assembly against the top brace to prevent rotation.
Over the last few years I have done that but as I glue the back first I access the gap through the mortise for the fretboard tenon. Wedge in some Mahogany tightly and hit it with thin CA to anchor.
I have watched a few of these guitars that are local the last few years and this appears to have solved the problem. They still move a little more initially than a glued extension or dovetail but the long term settling does not seem to be happening. An initial neck set with a straightedge 1/16” over the bridge on an unfretted board seems adequate to deal with the initial movement.
If any of you are experiencing similar problems with the Bourgeois style joint this seem to work. The other option is switching designs like the joint Trevor describes which I considered.
The pictures show the gap with a light in the guitar and the filler strip.
