Well, I just almost-finished the first install, but I forgot about the extra height when I bought supplies, and the supply hose is a couple inches too short. Like all of my big projects, this one was started on a holiday, so I won't be able to buy another one until tomorrow (aka Black Friday). I may have one in my supply of parts, but god knows where those might be in the 30 or 40 boxes yet to be unpacked. I just poured some water in the bowl to fill the trap and will finish it tomorrow. Stay tuned.
I did inspect the waste plumbing with the old toilet off, and discovered that the bend under the closet flange doesn't go directly back toward the other toilet, but hangs an immediate left toward the east wall of the bathroom. I suspect the opposing toilet will do something similar, but where they're headed, and why they might go off parallel to each other, is anyone's guess. I'm hoping, though, that they'll be far enough apart as the water flows so as not to affect each other.
Not being intimately familiar with every pipe and wire in the house is a new and unpleasant experience for me.