Lemme try something: Is the physical location of the water heater the closest to the outlet you'd think is first?
Is the piping the closest given the footage of the actual piping run? What race track are you giving it? I've seen it while crawling under a rental where it runs all over the place and the last place the hot water hits is close to the water heater. I'd crawl under there and just stare for a bit. Yes, take account for the shower head flow rates but you can rule those out by taking both heads off, unify, and turn on the water and yell when one gets hot water first.
I'm not ruling out the mixing valve because it eventually gets to the temp, unless they're playing with the handles. Full hot and wait.
(Hard water is also something, clogging up nozzles and why you can just spin those heads off and holler to each other when hot water comes out first)