I like the idea of an anti-siphon valve leading from the auxillary tank.
My 1989 chevy truck had 2 fuel pumps. One was the pickup pump in the tank, the other was an in-frame pump located under the cab.
How about a T-valve leading from the auxillary tank to primary. Then when primary gets low, then open valve to drain auxillary into the primary. Close valve when finished. Could figure a way to locate the valve so that you would not have to get off the bike, though stopping and shutting off the engine might be a good thing.