I have both the 12 psi version Facet and Ford mechanical lift pump in parallel, with check valves keeping them from backfeeding each other. I should also mention I put a fuel filter/water separator on the frame rail so both pumps are pumping clean fuel.
The reason I have both:
I installed the Facet after all the site support but it will NOT keep up with the fuel demands of a maxed out IP.
It would hold 8psi at idle and 1 or 0 psi at WOT. This screwed up the timing and lost me performance and MPGs.
So I got a Ford mechanical and did the parallel thing. It holds 4-5 psi no matter the RPM or the fuel demands. IP's are calibrated at 5psi. Coincidence?
So I made the Facet turn on with the cold idle advance and get all the priming I might need without it running all the time. The combo works well but I wouldn't suggest it.
If you go e-pump get a strong one and use an adjustable regulator. Some pumps have them, if not use an external. That's likely the direction I'll have to go when I get a hotter IP someday! Though if we ever find a higher volume mechanical I would jump all over that.
Happy wrenching!
-Joshua