But the reason most two keyboard systems are so difficult is that the need to use controllers and switches and see the display on the lower one puts the two WAY too far apart.
On something like the P85, with nothing sticking up above the level of the keys, you basically rest the arranger directly on it, and you are looking at little more than two inches of manual separation... Maybe not QUITE as small as a B3, but OTOH, the B3's action is also very short, much shorter than a piano's, which will change the playing position no matter HOW close they get.
I'm pretty sure we are never going to get what we want here, in any case, so our best option is to look for the best compromise. Trouble is, without the arranger manufacturer coming out with the 'remote input' idea I outlined (really, so little to add to the OS, I don't see why it can't be added if enough of us want it), or without extensive programming of the lower keyboard and some way of triggering the patch changes remotely, none of this works...
And, in the end, I'm not really sure that bouncing between a weighted 88 and a featherweight 61 isn't going to disturb our carefully practiced touch. Especially if you go back and forth a lot, it might be better to have two keyboards not QUITE so disparate in touch, IMO. Sure, you JUST want to play piano on the lower, you are good to go. But have splits, and use it for a lot more than that, and you get back to the same issues that require you to use a lighter action now.
Personally, come to think of it, my ideal two manual setup would be a waterfall 61 at the top, and my G70's 76 at the bottom. But sadly, no-one makes a waterfall arranger anyway, and the arranger HAS to be on top... so I guess it might have to be the Nord Electro 73 waterfall on the bottom. Not perfect, IMO!
What a can of worms we've opened...