I think staged comparisons have limited usefulness. The aim there is to get something to sound as close as possible to something else. Usually by reverting to basic tones. I do not intend to use an OB-6 to emulate a Prophet 6 (or an OB-X for that matter). I am far more interested in what the OB-6 sounds like on its own, as an instrument, used to its full extent....
I regard the Prophet 6 + OB-6 as a single two-keyboard (organ style), multitimbral, polyphonic setup.
I would tend to agree with you as regards the potential of the P6 + OB6 combination. Someone said an OB6 was a waste of money if you already had a P6. this user had a Prophet6 and a Prophet X to name but two, very different I guess.
I guess it would depend on what kind of sounds you prefer to make and hear. I'm really into the analogue I'm English!) synth type sounds and I'm not bothered about sampled sounds. I couldn't afford anything like a Proplhet X anyway.
I got a P6 on a swap deal in January 2018 and I can't leave it alone. Previously, I was leaning towards an OB6 because I thought the presets sounded better in the demos but I think the Dave Smith team had more time to get the OB6 ready for NAMM 2016 compared to the P6 for NAMM 2015. So the OB6 got better presented at the outset in my view.
Now I have the P6, it has its imperfections or limitations but it is meant to be a constrained design as Dave said, like the original Prophet-5. I keep finding sounds that I love when I play it. The modern additions, especially the arpeggiator, effects and inverted envelope amount capability (which is a never mentioned addition but I think it's great) seem to make the instrument much easier to play. I would rather have had 5 octaves though. That aid, the wonderful feel of the controls and the proportions of the P6 fit perfectly to a 4 octave instrument which is compact and neat, much less bulky than a P5 let alone and OB8, JP8, etc.
I have limited funds so maybe I will go for some recording gear instead of another synth - but I will add an OB6 if I can, because I love the sounds that I have heard from the demos and I am highly accustomed to the P6, which is its sister machine.
I would agree with your description of the P6/OB6 combination as being like a 2 manual instrument. I prefer this to a poly chain since I hate using cables and just love to play the things. Having the two as stand alone completely independent units is attractive to me.
As for staged comparisons, you have the limitation that you can really compare only kind of vanilla synth sounds that are common to many machines and is that really so useful? It's where they diverge that is more interesting. I mean a much better comparison would be for someone to create and play sounds on the OB6 that you in no way can do on the P6 - and vice versa. That would be a better guide as to which instrument to choose first.