In a Millenium interview with Ion Hazzikostas, he revealed that players will be able to equip their Covenant Legendary plus any other legendary at the same time. In addition, the new Legendary Currency no longer comes from Torghast, but from Zereth Mortis instead.
Millenium Interview with Ion Hazzikostas The original post is in French, but we've included the Google Translated version below. The summary:
Two Legendaries will be coming, but you can only equip your Covenant Legendary and one. Read more on wowhead.com