Hi. Welcome to the forum.
A D85 is one of the old school Yamahas, and could make some good sounds.
The D85 predates MIDI by some way. There were retrofit MIDI kits for some of the following generation of Yamahas, but I don't think you'll be easily able to Midify the D85. The Drum unit synch you're after would also be via MIDI so the same thing applies.
Adding a second D85? You'd need to slave it, and that would also be via MIDI. I think you see where this is going. Hardwiring it would be darned complex, if it's possible at all.
Modifiying the outputs? Might be possible, but you'd have to be pretty skilled to do this, again it might not be possible.
You're asking the D85 for things that either weren't thought of at the time, like MIDI, or not envisaged as being needed, like separate divisional outputs.
Seems to me these Electones bridge the gap between organ and synthesizer. You're right, they do. This was the time when organs were becoming orchestral and the distinction between the two instruments became blurred.
As for using it in studio, go ahead, do. You don't really need to MIDI it for this. I was recording multitrack albums on organs of this era when we were still using 2" tape for 16-track recorders. No synching, no slaving. You just played live, thenoverdubbed live, with all the natural variations and nuances in playing that occur. The music was all the better for this. Too much music is sequenced, not played these days. Go ahead, enjoy the great sounds you've got under your fingers.