Hello Ej: I didn’t want to assume the shower heads were identical in output. Also, the 15 year old heater makes the dip tube suspect. The difference in length of shower isn’t huge. The time it takes for things to cool down seems long to me if caused only by a cross connection. I’d expect cooler sooner than six minutes.
So many questions make me want to round up a little more info so I’m sure we have the problem surrounded. It would be nice to hear back from rasnickj and see if some testing has been done. A simple test would be to feel the hot outlet when getting cool water from a tap.
By the bye, I got to install my first Vertex equivalent from State today. It’s so technical …. makes me feel like such a codger 🙂