I have rigged them before where I took a small water feed line and routed it under the tailpipes, thru the transom rings and up into a small inlet in the back of the muffler. It was enough feed to keep them cool. Unless your cams have ridiculous overlap, it should be far enough away to not suffer reversion. I've also seen people drill into the inside liner of the tail to dump water into the exhaust stream.
The double-barrel does present some barriers but maybe the water feed idea might open up some ideas for you.