yeah.. double threaded.. the top is right-hand and the bottom is left.
first time i saw one of these things all i could think is .. you better not lose this sucker. i found a couple parts bikes but no tanks.. and no petcocks..
Try Honda. Thats worth a phone call .. or drop in and bug the crap out of the parts guy.. Tank/petcock nuts are probably the same on all their mopeds and scooters, regardless of year or style.. My guess is some brand new smaller bike with a tank/petcock uses that exact same nut. Ask around in the service dept in they will let you in there. Calling Service instead of parts should hook you up with an actual mechanic who might feel some sympathy.
My local Honda dealer is not inclined to lift a finger if the bike/year/part number is not listed as available in his computer .. your guy might be different.