I thought I would follow up here just in the nature of better understanding the layout of my initial post. I have since completed it, with both bathrooms tiled and toilets installed, so I can describe how it's working.
The toilets are both Kohler 1.28 gpf models. The plumbing shown above the black combo wye above is 4" PVC. The plumbing shown below the black combo wye is 50-year old 4" cast iron.
Flushing the toilet on the left does not affect the toilet on the right.
Flushing the toilet on the right quickly by pressing the lever and releasing will bounce the water around in the toilet on the left. Flushing the toilet on the right by pressing the lever and holding it until it empties the tank, will siphon about 2/3 of the water from the toilet on the left. With 1/3 of the water left in the left toilet, any amount of continued flushing (long or short flushes) from the right toilet will move the water slightly, but will not continue to siphon it. The trap seal never is broken, but it will become lower after a big flush, as I just mentioned.
I would assume that practically, this is not a problem. If I had to do it again, I would have installed another vent for the rightmost toilet, because it's somewhat annoying to go #2 on a toilet with about 1 - 2 pints of water in the bowl.