The point isn't that you can man both of them, though if you have crew and the water is filling the cabin... both would probably be in use.
The reason is if you have to be inside the boat for any reason—it is blowing 80 mph... there's golf-ball sized hail, etc—you can still pump out the bilge—which isn't possible if the only high-capacity bilge pump is only in the cockpit.
Likewise, if the only high-capacity manual pump you've got is in the cabin, and you need to be sailing the boat—say trying to claw off a lee shore that is going to kill you—you can still be pumping out the boat and helping the electrical ones out...
From what I've seen, a good high-capacity manual bilge pump beats most of the electric bilge pumps by an order of magnitude. For instance, a Whale Mk V, which isn't the highest capacity manual diaphragm bilge pump, does about 15 gallons per minute, or 900 GPH... with up to about a 15' head. A really high capacity electric bilge pump, like a Rule 3700 won't do much more than that if you've got any sort of head on it—as the 3700 GPH rating is for ZERO HEAD—and if you've got it in the bottom of the bilge and it has to pump it up six or seven feet to the top of the anti-siphon loop before it goes out the through-hull, it's probably only doing about 900-1200 GPH, but eating serious amps to do it.
BTW, the reason I compared the Rule 3700 with the Whale Mk V is that both use a 1.5" diameter hose, so should be roughly comparable in capacity, in theory.
I have one electric pump down in the bilge and one manual working from the cockpit. I like your recommendation to also have a manual one inside the cabin too but Im not sure if Im soloing I could use both at once.
So, what does everyone think about the shorting issue when flooded? Seems possible or likely? And all these boats with batteries that weve seen under the sole are just unaware or playing the odds? Am I worrying about something that is rare enough to ignore considering the benefits of the low batteries?