Being a former motorcycle person myself, I can't say I've tried carrying my shotguns while on the bike, but it's an interesting concept.
Doubles are a good idea, but how about two semi-autos? Reloading while on a cycle must be hell -- you'd have four shots total before you'd have to recharge the scatterguns. Maybe two gas-operated semi-autos (don't want an inertia operated gun if you're going to use pistolgrip-only shotguns) with Pachmayr pistol grips?
The 20 gauge is a good idea, I'd hate to fire any 12 gauge in pistol-grip only configuration.
Justin
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Justin T. Huang, Esq.
late of Kennett Square, Pennsylvania