You need a tank transfer valve, not a shutoff valve. Ford makes one
that is good, or try an autoparts store. Find out how they work before
trying to hook it up (IOW, don't guess at the wiring...)

OK, from the factory. I'll use Ford as an example.

There are two tanks. Each has a fuel sender, and  a pickup. Each tank
may also have a pump (especially if gas).

Both tanks go to a transfer valve assembly (mounted on frame rail
usually). This valve switches between the two tanks. Some valves are
reverse polarity switching (Fords IIRC), others have seperate circuits
for each tank.

The dash switch switches the tank transfer valve, and the fuel sender
to the gauge.

I suspect on the newest vehicles the TCM/ECM also gets involved.

Were I doing this, I'd see if I could find a *clean* (not rusted to
hell and back) doaner vehicle (similar model to what I had) with dual
tank setup. I'd then grab the plumbing (valve) and as much of the
electtrical as I could.

New from a dealer these valves usually cost about $120 (US $) or so.

You *could* do it totally by yourself, with a pair of 12V solenoid
valves, one for each tank. Energize the valve for the tank you want to
use. Down side, is that you have to use quality parts so they hold up.
You must also take into consideration that most engines circulate fuel
back to the tank and that fuel must go to the tank it came from!
That's another set of valves.

If this doesn't give yuo enough information, I'd suggest going to a
good library and seeing if they have any auto-repair manuals. Look up
a dual tank setup and that should give you both the electrical side,
and the plumbing side.

If you are totally nuts (and don't ask me to ride in your truck!) you
could do it with two manual valves mounted outside the trans tunnel
with the handles sticking up into the interior. Don't put the valves
or fuel lines into the cab! This would be the cheapest way, but would
require you do the management of tanks yourself.

I still recommend using factory type parts if you can. The factory
valve includes contacts to do the fuel gauge switch. The switch to the
factory valve handles the management of the two fuel pumps. If you use
a six way solenoid valve make sure it is continous duty, and rated to
handle fuel (most are, but check).