We never recommend running both an electric and mechanical at the same time. Pushing fuel from an electric thru the mechanical will damage the mechancial lift pump diaphram. then you leak fuel out of the ground and usually into the oil pan. Going into the oil pan quickly dilutes the oil and ruins the engine bearings. This is the reasons not to run that way. An electric lift pump as recommended here is one of the very best modifications we can do for our engines. It saves the starter from less cranking to get fuel pressure up. It saves the batteries from long cranks because you have a fuel leak down issue too. Its not a band aid repair but a good upgrade that saves in the long run.