I think he is using the term co-promotion literally. Like, "Strikeforce/EliteXC present..." or "Strikeforce/M1 Global present..." Bellator hasn't had any cards at all that feature another orgs name on the bill, as far as I know.
But, RIZIN has used fighters from many different orgs, as you said. It is more like borrowing though. Even recently they had a fight that was Deep champ vs Pancrase champ. But neither orgs name was on the bill. I believe Coker has been talking recently of doing a true co-promotion between RIZIN and Bellator. He needs to get it done, it would be great.