I've recently done this and it works fine. The only problem I had was that I positioned my pumps too high for my flat tank, but once I dropped the pumps to the level of the van floor (pressure switch down), it's worked perfectly ever since.
I just use a "T" piece to split the single pipe into two to feed to either pump.
No flow problems happen and the battery is fine.
A little tip is to use some electrical tape to colour code the varistreams and the hose reel that it uses, because I'd be working away somewhere and Wor Lass would muddle up the hose reels and try to reel mine in, instead of hers.
Or if I was working by myself, I'd turn one flow controller on, and then walk off with the hose belonging to the other flow controller that wasn't turned on.
