I think Magna Carta Holy Grail was initially supposed to be the joint album. It's just a hypothesis, but given the album title, some of the songs (specifically "Holy Grail", "Jay Z Blue", "Part II", "Beach Is Better" and "La Familia") and Beyonce has vocals on several songs. Then there's the fact that Jay recorded MCHG during the same period as Beyonce was working on her self titled album. It was the same studio with a lot of the same features and producers (Timbo, Pharrell, Justin Timberlake, Frank Ocean).