I have been told this won't work but I'm interested more into why. They make adjustable fuel pressure safest switches like http://www.ebay.com/itm/111127410798?redirect=mobile that are made to turn of when the fuel pressure drops bellow a preset level. Why can't you use that to turn on a second pump if wired correctly?

 

Because once the pressure drops it is already too late.

Bypass regulators continuously bypass fuel. If the pressure is dropping, the regulator is not bypassing any fuel & no longer in control.

You need to enable the second pump before you run out of fuel from the first pump.