Talking about stepping, +/- 1 stepping should be fine.
Windows should be running multi-processor core already, since you have a dual-core. By adding a 2nd processor, Windows should see 4 cores.
Windows license is based on physical CPU slots, not cores. For example, if your Windows supports 2 processors, you can use two 4-core processors with HT enabled, and you will see all 16 processors in Task Manager.