Mehrunes Dagon seeks destruction, Molag Bal wishes enslavement. It is said that Molag Bal is not inherently evil, while Mehrunes Dagon seeks the utter destruction of Nirn. Since one wishes utter destruction while the other wishes submission, I cannot see the two working together to a single goal.
Supposedly Sithis communicates with the Night Mother, but when Thelos ascended he changed Cyrodiil, but you never see nor hear from him since.
Very sound reason. Thanks for your reply. And because of this distinction of destruction and enslavement, what do you think the relationship Dagon and Bal is like? Or we just have no knowledge about that?