I would add to Ken's reply that you could also re-program some of the InfinityBox outputs to drive dedicated pumps, thereby eliminating the use of relays. The outputs are more expensive than relays of course, but are much more programmable if needed.
For example, assuming you had a LP and HP pump, you could program the LP pump to come on 2 seconds after the HP pump (it wouldn't need to come on immediately if the HP pump was pulling from the swirl pot). This would reduce any voltage drop to the HP pump that arose from multiple pumps or other loads at start.
With a little more sophistication, you could use an Arduino board and an electric pressure sensor to read fuel pressure, and feed back a 0-5v signal to InfinityBox which could then use pulse width modulation based on the 05v signal to control motor speed so the fuel pressure was always constant, without regard to input voltage variations from large loads. KenneBell has a pump controller that does this for much more money.
Alternatively, you could actually measure the pump loads, and if they were small enough, accounting for startup loads, you could just wire them in parallel, saving all the effort of relays, programming, etc. That's the simplest solution, and one that a lot of street-driven SLCs I know about use.
