There are many systems, many machines, many different ways to do the 2-stepping for fleet washing.
Something that a couple friends told me was that with the more gpm's you run, the faster you wash so if you upgrade to the 5.5 then 8gpm, you will wash faster and faster along with rinsing a lot faster.
You can use systems that you can buy like the kind that run chemicals through the pump that have 2 hoses running to the gun, you can get the Bioclean system, you can use downstream injectors, you can run soap through the pump and downstream the acid and manually turn the soap on/off, use a remote, walk back and forth to the truck/trailer, you can run both the soap/acid through the pump and switch like the above, you can use pump up sprayers for the acid, etc....
There are so many ways to do this, a good idea would be find someone not in your city and go train with them to see how they do it and learn what you like/dislike and then find another company that does it another way and then you can figure out what way/system you want to go with.
There are just so many options these days, hard to decide without seeing them in action.