Do you switch between both watches regularly? Are they both paired with your iPhone? If so, the data that shows in the Activity app on your phone should be accesssible and show tracking from both watches from whenever worn.
If not...
It is my understanding that information accrued from multiple Apple Watches that are paired with your iPhone will retain the information in your Activity app.
If not..
You may want to create an encrypted backup through iTunes and unpair the S1, and then re-pair the S2 and all data should be viable and accessible.