Dave, If you are thinking of pulling the fuel from a on board C Dory tank--it will not work.
Warren. The way the system works is that there is a vacuum formed in the Honda generator tank, as the fuel runs by gravity to the carburator which is close to the bottom of the case. The "extra" fuel comes over from the tank, pulled by this vacuum. If the auxillary tank is lower by 6" than the generator tank, it most likely will not work. If the tank is higher by 6" that is fine, but I would not want it much higher. Remember that you are pulling fuel from the bottom of the extra tank--and that is aaround 6" below the top of the tank. Not not truely a siphon--but the same principle--the tank must be close to the level of the fuel tank in the Honda. I say that it technically is a siphon because the ultimate source of the use of the fuel (the carburator) is lower than the fuel tank and there is a closed system of fuel.
The EU 2000 will not run a 8300 BTU air conditioner all night without an auxillary tank--I have too many nights aboard to prove that--and that is why we went with an auxillary 6 gallon tank early in the game. We carry extra gas for both the generator (no oil) and for the outboard (2 stroke requires oil addition). I would not try and pour from a 6 gallon outboard tank--but you can use the squeeze bulb to transfer small amounts of fuel.