True Dual Exhaust 1999 Silverado

Does anyone know how to put true dual exhaust on without getting a code? I would like 3" flowmasters with an X pipe can it be done? Ive seen things on Ebay that plug into your O2 sen. so U don't get the code but don't know if they work. I have installed a K&N cold air intake, edelbrock headers, programmed the computer so exhaust is the next step. thanx for anybodys help.