We haven't fitted a second filter but we've been near woop woop and the fuel isn't all that bad. We bought some fuel from Kings Creek Station and we were thinking "this could get nasty, being stuck on Luritja Rd 300km from anywhere". Bowsers are under the roof located at 24°24'18.95"S 131°49'3.55"E if you've got Google Earth or you can
click here to get a Google Street View from the road.
Now that place was flogging diesel for 175.6cpl when the rest of the region was selling for about 166.9 and Port Augusta had it for 123.9cpl. I thought I'd see burnt chips and battered fish coming out of the pump but it was all as clean as a whistle.
If you suspect you may encounter really bad fuel, a second filter might be a good idea but once it starts blocking up your pump may struggle to deliver.
I've thought about adding a booster pump and maybe even an intermediate holding tank that holds about 1 litre of fuel. You bring fuel up to the holding tank through your new filter and then the original pump & filter takes from the holding tank. Excess fuel is returned to the main tank. Supply to the holding tank is restricted either by float valve or simply through pressure, with a back-flow that returns fuel to the main tank.
It's overkill, I think, and over-engineering the problem, but if we do have bad fuel in places this might alleviate the risks.
Certainly a second filter won't hurt, but I recommend carrying a spare or two.