Hi Tosh,
I think you have a Variflow which is a different controller than the Varistream that we have always used.
Our new digital ones don't have the provision of an additional inbuilt charger circuit connected to your van battery, hence the reason for us having separate split charge relays on our vans.
According to Williamson Pumps who supply the Varistream and Shurflo pumps, the electronic pressure switch of the Varistream is already set up and so compatable for use with a 100psi Shurflo pump, so it is pretty easy to connect it up and use it without too much trouble.
I have a 2 operator setup on my van for the occassions my wife or son work with me, 1 pump and controller per operator, and running them has never been an issue.
I once saw a setup with one controller, one pump and then split for 2 operators - don't know how long that setup would last as the pump was working flat out and cycling like made when only one brush was working.
Picture of Varistream controller I use attached.
Spruce