[Commander]

Big Jaguars have used dual tank setups for decades. From a Jag helpsite:

Fuel Plumbing.

D Jetronic introduced the recirculating fuel system used on just about every electronic system since until the recent advent of non-recirculating systems. The pump draws fuel from the tank and delivers it to the fuel rail(s) (the pipe which feeds the injectors) having an exit to a pressure regulator which maintains constant pressure at the injectors and spills excess fuel into a return pipe to the fuel tank. The flow rate in the system is always in excess of the maximum that could ever be required by the engine. D Jetronic regulators are adjustable but should be set to the correct pressure.

The V12 installation uses separate fuel rails and regulators for each cylinder bank but the two are linked and it is important for both of the regulators to be correctly adjusted so that the slightest backing-off on either will immediately cause a pressure reduction. Twin tanks in XJ12 saloons have a system of solenoid valves controlled via the tank selector switch to direct recirculating fuel to the tank from which it originated. Any air entering the system when a tank runs out is purged by a bleed valve located in the spare wheel well.

There is a non-return valve built into the pump, and another attached to the air bleed unit on saloons, which are intended to retain pressure in the rails after switching off with the intention of preventing vapour lock during a hot restart. Unfortunately this can still happen, partly because of the relatively low fuel pressure, and was a minor irritation associated with the D Jetronic system.

On saloons the solenoid operated change-over valves, which ensure fuel is drawn from, and returned to, the selected tank, can fail, leaving one tank unselectable or rapidly draining one tank into the other. If the tanks are more than half full in total, after filling one tank the remainder will be ejected through the vent system. Fortunately the valves are easy to change, later cars having a separate return valve to each tank in the rear wheel arches, earlier cars having a single three way valve in the boot well, along with the three way selection valve.

So there are parts available from Jaguar service centers to do exactly what you want to do. The question is, do you want Jaguar electronics in your car?