It depends on what sort of control you want, for example are you expecting to control the pressure at a constant pressure i.e. PID control, or is it just if the pressure falls below a certain level start the second pump to keep it above the minimum setpoint.
The cost difference in the two aproaches is significant, as well as the control limits.
It could be that two pumps one duty & one boost if needed does not give good control of pressure there is also a danger of over pressurising the system or the standby pump may be switcing on/off above it's duty cycle.
The ideal way is to have the pumps controlled by VFD's (or at least one of them), then feedback the pressure to an analogue input.
But this requires the addition of VFD's & Analogue card whereas simple on/off control of the secondary pump does not need a VFD.
I think more detail is needed to give you answers that pertain to your application for example a diagram of the system as you see it, control type & feedback.