I used to do the two swivels. I had two upper ones start to leak in the span of a couple days and didn't have any spares, so I took them out until my order came in. I can't tell a difference in the ones with two and the ones with one swivel at the gun, and I do a lot more turning around while cleaning bays than customers do washing.
Mosmatic are good, but if I were to run just one swivel I'd put it at the boom between the bay hose and swivel saver. They last a lot longer there without the wear from being pulled on sideways like they do at the gun, plus they get really beat up and ugly at the gun. I just use a brass Adams swivel at the gun. There's probably something better, but I take the Adams apart and put marine grade wheel bearing grease in it and they last a couple years and swivel easily.
with a 3/8" coupler and off of that we have a 12' - 3/8" hose with another 3/8" ball bearing swivel at the gun. The ball bearing swivels last a long, long time, we very seldom ever have the replace one. .
Mosmatic SWM32564 is the best gun swivel I have ever used. I have ordered the cheaper swivels for years and finally ordered one of the mosmatic to try. I ordered one for every bay as soon as I tried the first one. They are pricey at $25 but you can feel the difference. My 2 cents.