I agree that the "double hip" is a training method. I can see its use against shields and bags and in other "static" environments.
However kata is, to me, a tool for grooving combinations in a dynamic
setting. I think it is very unlikely that you'll ever do a "double hip" in combat. Rather your hip should be set up by a previous parry etc. - and you'll go from whatever position your hip happens to be in (rather than some ideal "power" position).
In this respect your hip use is no different from any other "loading" - eg. loading arms for punches and strikes, or legs for kicks. You should learn to move so that you end up in the right (or at least an appropriate) position. You wouldn't artificially load your hip, no more than you would interrupt a flow of movement to load your punch into a chamber
. Instead, just as chambers on one arm should take place while the other is in action, so your hip should load during the preceding movement.
Accordingly this "double hip" strikes me as a "wobble" that is akin to being "wrong-footed" in sparring - it cuts against your flow of movement in an attempt to create an "ideal" power load.
Simultaneously it assumes your hip is not already sufficiently loaded for the required technique. Karate is a civilian defence system
; ie. it has the central aim of "not getting hit". It is not a sport where you're looking to "win" - perhaps by scoring a "knockout punch". A civilian defence system doesn't try to maximise its power on every single technique. It focuses on safety first.
In the case of sochin kata, the punches from neko ashi dachi are akin to the opening punches in goju's seisan - they are "snap-like" punches - not full hip "power" blows. Any attempt to force them into the "power" mould misconceives their function and robs the student of practising very useful, situation-appropriate self-defence skills (ie. kizami zuki). The fact that neko ashi dachi is not usually seen as a platform for the delivery of power punches (except where it is used to lunge into zenkutsu dachi) is, I feel, supportive of, and consistent with, my argument.
So rather than being a development of karate, I see this version of sochin as tangential at best.