If a supervisor needs to be assigned to the job of another supervisor, there are a few ways of doing that that can be decided by the business. This reassignment process needs to be done at the Job level with no feature currently to 'bulk' assign one Supervisors jobs to another Supervisor.
Set new Supervisor as Supervisor 2 in Management
This has the effect of the Job and all Notifications also being sent to Supervisor 1 in the event they return to work, and after the secondment simply removing Supervisor 2's name will finish things up.
Change Supervisor 1 to the new supervisor.
Changing Supervisor 1 name will completely assign the Job to the new supervisor, meaning the previous Supervisor will not retain any connection to the job, administrative or otherwise. The new Supervisor is now completely responsible for the Job, and a note will need to be made of who the Previous Supervisor was if they plan to return the job to them later.