Reporting question for Speaker - Celestion, 12", G12M-65 Creamback, 65W

Product:
Speaker - Celestion, 12", G12M-65 Creamback, 65W

Question:
If I have this 16ohm speaker in my combo amp and I want to add an extension speaker, does it have to be 16ohms also? Or can I use an 8ohm speaker with it? What is the best way to use it?