We knew from the release announcement that two legendaries were going to be limited behind one Covenenant specific and one other Legendary, and today we now know more details that will make the swapping process easier.
Players will be able to obtain a new legendary belt from the Enlightened Brokers in Zereth Mortis, which will be empowered with the Covenant Runecarving ability, and will change power each time a player switches covenant, so there is no need to craft 4 legendaries.
If players do not wish to use this belt, a new Runecarving Memory will be available to craft this effect onto another item slot, but these new slots have not been determined yet.
How Double Legendary Items Will Work In World Of Warcraft: Eternity's End
This new belt and the other legendary will need to be crafted around tier sets, as they may potentially cross over, and if a player is unlucky enough to not loot separate pieces they temporarily may need to change up the slot.
Thankfully, upgrading legendaries in Eternity's End will be much simpler as they will just require a new currency called "Cosmic Flux" which is awarded from content players are already likely doing such as Raiding, PvP, Mythic+, Torghast, and World Quests.
This new currency will also be used to upgrade normal items to tier sets, for those who were unlucky with drops, or do not raid in Eternity's End.
This is a good change, although some players are still disappointed that they are locked into playing one normal legendary and one Covenant specific, and Blizzard may be unlikely to change that before Eternity's End despite player feedback.