Co-teachers can be added to your class using a teacher join-code.
Step by step: Adding a second teacher
1) Navigate to your class Users page (Settings->Users)
2) Click the "Join-Codes" button
3) Enable the "Teacher" code.
4) The teacher(s) will navigate to your Kidblog class URL, click the "Join" button, and enter the code to join the class. They will be prompted to create a Kidblog account or login (if they have an existing account).