Nick:
"So in both of these, and the version that the TreeSpyder suggested, they did advance up the rope easily, and they did lock up tight. But in both variations, I noticed something. After loading, they did NOT want to release their grip to continue ascending. It took a lot of finicking to "break" the prusik in the styles that Glen and Spydie suggest."
But isn't the objective of the hitch to merely act as a backup, should one of the ascender cams fail? It seems you would only have to worry about "grip lock" of the hitch if one of the cams did indeed fail, and that would be only under extraordinary circumstances. In which case, you could either figure eight or munter hitch down to the ground.
I don't see the point in searching for a hitch that would allow an easy release from the ropes if the whole point of its being is as a backup. Of course, it would be nice if such a hitch existed, but, I think it would be unnecessarily complicated, thus defeating the purpose of efficient climbing.
It seems to me that the best backup to the double ascender is the simplest, easiest-to-tie hitch, that slides easy up the ropes, and always grips the rope should an ascender cam fail, regardless of it's "griplock" on the ropes.
And I agree with Glen, the chances of both cams failing at once are so extraordinarily unlikely as to be considered as nil.