Hi,
This may happen if the first does not finish within the time span given for it to complete. If the second backup job was suppose to start after the first job completion as per a schedule then ensure that you have given ample time for the first job to complete. The schedules should not clash with each other that means the first one should not go beyond the starting time of the second job. this may also happen if the device used for al the backup is same and the device is occupied at that same time with the first job.
You can also use the automatic cancellation option to cancel the job if it does not finish within the time slot specified.
We hope this will help.