According to HerHusband :
Is there some way an additional pressure tank can be added at the pumphouse
that will ensure that one of the tanks is always fully pressurized? I would
think just adding a 2nd tank would still leave the possibility that both
tanks could be near their empty state when the power goes off.

Add a second tank with a simple checkvalve. Whenever the system pressure
rises above the second tank's pressure, it'll fill. But when you draw water,
the second tank is prevented from participating by the checkvalve preventing
backflow.

When you lose power and the primary tank goes empty, you switch your
water feed to the other tank.

The "who is connected to the water feed now" valving need be nothing more
than a two-handle Y valve, or two separate ball valves.

[You want to disconnect the primary when you switch to the secondary to
avoid waste of air pressure repressurizing the empty tank.]

I'd do it symetrically, so both tanks have checkvalves on the "fill side",
and both tanks have ball valves on the "load side". So you can flip which
is which for maintenance etc.

Depending on your well, you may want to "flip" them every week or three to
ensure that the "secondary" doesn't go stale/breed bacteria. Professional
plumbers may want to comment here about long term storage of well water...

You could prevent this being an issue by, instead of using a
checkvalve, use an electric valve on the secondary, so when power shuts off,
the valve closes. Then both tanks will be in full operation during power-on,
and one gets disconnected and reserved for backup when power goes off.

You could probably use an irrigation valve (24V), which are only open when
they get power (from a small transformer in the pump shed). They're only
about 20-25 bucks at Home Depot for 1" units, and they do do high flow rates.
These valves may not like reverse flow and "flap" however.

Alternately, bite the bullet and put a monstrously huge tank on stilts -
no longer need to pressurize it.

I knew there was a use for water towers here somewhere ;-)
--
Chris Lewis, Una confibula non set est
It's not just anyone who gets a Starship Cruiser class named after them.