First of all, is it possible/practical?
I've read of people upgrading to a bigger inline fuel pump while leaving their in-tank pump in place... running to pumps simultaneously.

Im trying to cure my long crank problem. I have to turn the key as soon as I hear the pump stop pumping, before the pressure goes back down to zero. I have to get a new pump regardless ( I'm looking at the Walbro GSL392).
I'm currently using an MSD inline that isn't holding pressure. Once the car is running, it's fine-- it's just starting is the problem. The fuel leaks back to the tank through the pump when it's not pumping. It's supposed to hold pressure right?

So my last question is, When I get my Walbro pump, can I install it in front or behind the existing MSD inline pump? Or should I just throw that one out?

 

Doesn't provide any benefit. Put one proper and sufficient pump in or by the tank, leave it at that.

Remove anything inoperative, like the stock in-tank if it is still in there.

 

Like five7kid said, you're systems only as strong as your weakest link. You could run two pumps parallel but in your case that wouldn't help.

 

Thanks for the response guys... this is what someone sent me: "doesnt really sound like a fuel pump problem. it sounds like the diaphram in your fuel pressure regulator is leaking down. you can check this by pulling the hose off the intake that runs to the regulator. hose is next to map sensor hose if it has fuel in it its bad.also you can put a fuel gauge on the fuel rail at the schrader valve to check pressure and do a leak down test at the same time. replace diaphram problem solved, money saved,jimmy"

I'm gonna check that before I buy the Walbro...