How can I add another Directory Admin on OpenDirectory
You can add a second Directory Admin by either using the Server.app GUI or via command line to your OD server.

GUI:


Open Server.app and enable View > Show System Accounts
In the sidebar choose Accounts > Users
Click the + button at the bottom to add a new user:



Choose Local Network Directory for Directory, enter all details and save the user.
Right-click the new user > Advanced Options... and add or modify further details:


Right-click the new user > Edit User... and remove all group memberships except Administrators, Open Directory Users and Open Directory Administrators




More or less without GUI:


Use a predefined/precreated ldif file and import it:

0x0A 0x5C 0x3A 0x2C dsRecTypeStandard:Users 11 dsAttrTypeStandard:Expire dsAttrTypeStandard:Change dsAttrTypeStandard:FirstName dsAttrTypeStandard:UserShell dsAttrTypeStandard:GeneratedUID dsAttrTypeStandard:PrimaryGroupID dsAttrTypeStandard:RecordName dsAttrTypeStandard:UniqueID dsAttrTypeStandard:LastName dsAttrTypeStandard:RealName dsAttrTypeStandard:NFSHomeDirectory
0:0:Directory:/bin/tcsh:24DCA574-8E9D-4F1A-84B5-79A18882F5CF:20:diradmin3:1002:Administrator3:Directory Administrator3:/private/var/root


The UUID (24DCA574-8E9D...) has been generated with uuidgen. Also change UID, all names and other details.
Import the file with:

dsimport /Users/user/Desktop/file.ldif /LDAPv3/127.0.0.1 O -u diradmin -p &lt;diradmin_password&gt;

add/modify password and group memberships of the new diradmin3 with tools like dscl or the GUI.