There's no emissions testing where I'm from, so I couldn't answer you with specifics. I would assume that you need two of everything. My 1990 Nissan 300ZX (miss that car...) was true dual from the factory and had 2 of EVERYTHING from the intakes to the exhaust. Two air intakes, two throttle bodies, Four O2 sensors, two cats, two resonators, two mufflers... It was passing emissions in California where they're supposed to be super strict, so I suppose if you needed to pass emissions, do the 2 of everything thing. For the exhaust that is.
Another option is to ask around and find a inspection station that is a little lax with their requirements. Your choice.
Gas mileage with my setup -- no cats, two Thrush glasspacks, true duals -- is around 16-18 city. Thats on a 1991 RCSB with a 305. I think the biggest improvement for HP and gas mileage was replacing all the old worn out stuff with new parts, (cap and rotor, plugs, wires, air filter, fuel filter, etc...), so if you need better mileage, try going that replacing things first. Plus, the I have the Hypertech Powercharger which helps out too at the higher RPM's.