If you start Seconds after another workout app, the workout session of the other app will be cancelled. This is all done automatically by the system. If one app start a workout session and there is already a workout session active, the active workout session gets cancelled.
Apple Watch runs one workout session at a time. If a second workout starts while your workout is running, your HKWorkoutSessionDelegate object receives an errorAnotherWorkoutSessionStarted error, and your session ends.