It is true, configuring a vehicle's exhaust design to anything other then how it originally passed Federal and California emissions testing will cause a vehicle to fail the smog check. The vehicle will be considered having a "tampered" emissions system. It is very clear that adding catalytic converters to an older model vehicle or one which never required one, may effectively reduces emissions, however due to the vehicle never having a catalytic converter in its original exhaust system, it will not allow its placement or addition. Converting to a dual exhaust system from a single exhaust system falls into this same category... tamper. A smog check station, whether test only or test & repair, must fail a vehicle when such a problem is identified.
One option you might have is an inspection by the State's smog check referee's office. They might be able to inspect your vehicle and issue you documentation indicating the vehicle passed the smog check with it's modification and further inspections should allow for the dual exhaust configuration. It must be noted that this smog check process is reserved for engine changes, but non-the-less, it wouldn't hurt to speak with a referee and perhaps have them take a look at your truck's dual cat setup. You may contact the Smog Check Referee's Office at (800) 622-7733.
