There is no way to merge accounts.
But you can export workouts from the other account and import them to the other account.
To export workout
You need to go to www.sports-tracker.com and log in.
Choose a workout. On the top right corner you can find "EDIT", "EXPORT" and "DELETE"
Save the GPX file. Bulk exporting is not available.
To import workout
On the dashboard on Latest workouts on top right corner you find "+ add workout" and on the next page "IMPORT"
You can import 10 workout files at once. Supported file type is GPX