On the brodex system you by-pass the pressure switch on the pump. It is a Williamson Pump's Varistream unit.
If the Brodex tank is a one man system, then there is every possibility that the circuit board for the second pump controller isn't there. It would need to purchased as a spare part if it isn't, or wire in a new controller and secure it the the side of your van etc to drive the second pump.
If you do buy a circuit board from Brodex, then you will also need the connector block and wiring. I think its quite expensive.
Simon's advise is good to keep the pressure switch in the circuit loop, although I know of one case where doing this caused 'havoc' with the Varistream. This could have been a faulty connection or a faulty micro switch. Removing the P/S from the loop solved the problem.
Spring controllers recommend you leave the P/S in the loop, Williamson pumps recommend you don't.