You would treat it just like it was a non-Comcast address book. You need to export it as a .csv file, then import it into the secondary account. This is a much easier process than it used to be. Open the address book in your primary account, then select "Address book" in the left pane. Click on the "hamburger" icon that appears to the right of it. Select Export and follow the prompts. In the secondary account just reverse the process and click on "Import".