. I haven't seen one yet offered in double barreled models here either (SBS or O/U) but I do like the idea.
600+ of them on Guntrader at the moment in O/U alone . Double barrels are BIG here
http://www.guntrader.co.uk/Guns-For-Sale/results.php?fetch=Results&Filter[NewType][]=Shotgun&Filter[NewMechanism][]=Over%20and%20Under&Filter[Calibre][]=20%20gauge&perPage=30&sortField=Price&sortOrder=asc&skipNo=300