I'm thinking if your first watch was paired to the phone, and then you paired the S2 to the phone without doing anything to the first watch (no unpairing, no removing from your icloud account, etc.), then both watches should be paired to the phone.
WatchOS2 provided the ability to pair 2 watches to a single phone.
The phone will determine which watch is on your wrist if you set the toggle to "auto detect" in the watch app on your phone. Tap the watch icon to see.