I think there is a solution actually,
If you take the 'locking pins' out of the barrel then the tank will open when ANY key (that fits the slot) is inserted.
I've done this before on an ignition barrel so the bike could run with any Suzuki key. I'm sure it would work with the tank as well. The tank cap is really fiddly though. make sure you remove it from the tank first and work on a clean clear surface, you wouldn't want to drop a screw into the tank!! (like I did...
)
If this was cool with you then, you'd just select one tank to have the a dedicated key (i.e. the same as ignition/seat) and the other works with anything.
That said though, the extra time taken to swap the locks over is nothing compare to the time taken to swap tanks? half hour job?