Folio only supports one direct supervisor per employee. But Folio has 2 features that can give supervisor-like access to an employee's profile, goals, etc. These are Groups and Collaborations.
Folio allows you to set up a group and assign a Group Viewer. The Group Viewer has all the access to an employee's (member of the group) Profile, Goals, Notes, etc. as a supervisor with the same note creation abilities of a supervisor. This would be the best option if you would like an employee to have a "second supervisor."
Coaches and an Athletic Director would be a good example of where Folio Groups could be used. The Athletic Director could be made a Group Viewer of a "Coaches" group. Teachers who are also coaches could be added to the group. Then the AD would have the same visibility and note-creation abilities of the teacher's supervisor.
Collaborations are similar to Groups, but the visibility of the Collaborator is more limited and their note-creation ability can be restricted to specific note types.
Collaborators may be granted the authority to publish any of the Note Types. They can also view an employee's Profile and Goals. However, they cannot see an employee's Survey or any Notes between an employee and their Supervisors.
This is a quick answer to what may be a complex situation, so feel free to give Folio support a call at (800) 452-5078 Ext. 1 to discuss your specific situation. Folio support can also be emailed at email@example.com.