There are lot's of ways to put "twice pipes" on your banger. Headers are probably the easiest. Depending who's engine and what vintage there were any number of cast iron exhaust manifolds manufactured, some reproduced today. Then there's the old trick of cutting a hole in your existing cast manifold and welding in a second flange, but that's only marginally successful unless the guy doing the work understands preheating.

Here's one of the better sources for inline six hop up stuff http://www.cliffordperformance.com/home.htm