So what engine do you have? A new V8 is easy to make duals. An old 235 or 216 and you'll have to get a new / altered manifold or split the exhaust somewhere past the manifold.
Do you plan on running a muffler before the stacks? Wonder how noisy it would be with no muffler and stacks running straight up.
What size pipe are you going to run? A 3" pipe, dual stacks, on a 216... is NOT going to help your performance. A single 2" might, or a single 2.5" on a 235/261. A high reving 292 might like a 3", but that's pushing it.
A pipe (of the correct ID) needs to be long enough to allow exhuast gas momentum to help "pull" the gasses out of the chamber, but not so long that it restricts the air flow out of the chamber. It's a fine line, and I don't know of any certain way to determine it.
Old tractor pullers say to put a long straight pipe on, then run a crayon mark the entire length of the pipe. Run the engine till it gets completely warmed up. The location where the crayon mark has stopped melting is where the pipe should be cut. Sounds like voo doo to me, but maybe it works.
