I guess you didn't have to change fuel filters very often
I know their are people on here that are running pumps in series just trying to figure out a part number before I just go buy one and play with it.
I change my filters every year. Just fill them with fuel before you put them back together.
The problem with running them in series is that unless both are running, the one stopped can block the fuel flow. Just connect them in parallel and either can supply the fuel.