Both primary and secondary admin roles are the same except that the primary admin may invite other administrators. The secondary admin cannot invite another admin to join.  Assigning or inviting a role does not remove the role from the assigner.  

For instance:  When starting service, a primary admin is setup for all the congregation that subscribe.  The primary admin will thereafter likely want to invite a primary admin to each congregation so they can setup their own attendants emails for count, passwords, etc.  

The new admin for a congregation may also wish to invite a helper as a secondary admin.  A primary admin or secondary admin can view/edit the specific congregation where they have been invited to that role.

If the brother administrating is leaving or no longer wants the role of a the primary admin, he can invite another brother and then his role can be deleted.