one pump instantly dying is the best, as the engine just cuts out. it may lean out for a second but usually not long enough to do much damage.
the main reason two pumps are dangerous is if one dies and you still get enough flow from the first to run normally your not going to have any idea that a pump has died. which means you'll keep hitting peak power etc without knowing its running lean. for example, say you have 2 pumps that flow 200lph each (~400lph combined) and you need 250lph under full load. if 1 pump dies completely your motor will only be getting the 200lph from the other pump (assuming you have 1 way valves), leaving you 50lph short, this will result in a drop in fuel pressure and afr's going lean. you could drive it like this for ages without realising until its too late
as said earlier, a wideband and gauge would remove any doubt everytime you plant your foot and is relatively cheap insurance.