Principals can supervise assistant principals if given the "building lead" designation via the assignments tab. 


How to set a principal as a "building lead"...


As a School District Admin user, access the Assignments tab.  Select the appropriate principal and designate them as the "lead" principal for the building.  See screen grab.  Any other principal assigned to that building will then be able to select the "lead" principal when starting their NTPE observation.