I think it would be dangerous to say there's never been one, but I can't recall ever seeing one. I've seen and shot the odd single with a crossbolt behind the trigger, but never a double. There again, there have been all sorts of safety designs over the years - I've even seen double guns with grip safeties. However it strikes me that there are good reasons most doubles (SxS and O/U) have a tang safety. I've shot guns with cross-bolt safeties both in front of the trigger and behind, with a safety in front of the trigger which pushes forward, Greener side safeties and probably some others, and I far prefer a non-automatic tang safety for speed and convenience. YMMV
