Our trucks use GM's Passlock II system. The keys for the Passlock I and II systems are in no way different than your normal key. The only RF immobilizer system GM uses is in the newer Passlock III systems, which is not used in our trucks. These are the ones that have to be programmed.
The PKI and PKII systems use a resistance code from the ignition cylinder, rather than from the key like the older GM VATS system or a RF system in most other manufacturers' vehicles. So when you turn the ign cylinder to the crank position with the right key, the PKII module sends that resistance code in the ign cylinder to the BCM, which then decodes it and sends the signal to the PCM that it can start the vehicle.
So, all you need is for the second key to have the exact same cut. No programming required.