We've had a 2 man set up for the past 7 years and we use a 650 litre flat bed tank, sitting over the back axle, held down with a couple of heavy duty ratchet straps which are bolted with 2 inch steel plates/washers to the floor.
I've got a hose going from the bottom of the tank to the pump which then feeds out where I've got a y-piece going to 2 hose reels. There's an on/off switch on the wall but I don't have a varistream controller or similar and I'm not sure we need one, unless I'm missing something? With the 2 reels going the water flow is good, when one stops (we manually unplug it from the reel) the water flow is a lot faster, but still manageable. Both reels would tend to finish around the same time anyway, as the guys work towards each other and finish together, give or take the last window or 2. One hassle I do have is that the male pole hose connectors which attach to the reel seem to have changed profile over the years and they're a nightmare to take on and off. I'm thinking of putting a small section of standard hose in between with a standard female and male hozelock fitting on either end, to make it a bit easier.
I've attached a pic - it's a very basic set up, but it works for us. We have 2 IBC's in our yard with a tonne of purified/deionised water sitting in each of them, ready to fill each of the vans. If we run out of water during the day and we have to fill up with tap water at a customers house, we just attach the DI tank in the van for the rest of the day and then at the end of the day we drain the left over tap water and re-fill back at yard with purified/deionised water.
One good thing for us with this set up is that we can take the tank out fairly easily meaning we can use the van for other jobs. The van pictured has 2 side doors so I found it easier to mount the poles on the roof inside guttering downpipes.
We're getting a new van soon, so I'm looking at what other guys have set up in their vans. No point re-creating the wheel!
Cheers
Rob