Edit (11:35 AM): Ion Hazzikostas has also commented on this, noting that the double Legendary is the Covenant Legendary, plus any other Legendary.
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In an interview with Nga, Executive Producer John Hight confirms that the ability to equip two Legendaries is coming in Patch 9.2, with hints that Legendaries can be obtained outside of Torghast. The ability to equip two legendaries needs to be unlocked from the new Zone, but the unlock seems to be account-wide.