A question about detachments

I'm a bit confused about how detachments work in the game now. Is it possible to use two Dark Eldar detachments in an armylist as long as they are not the primary army? Was thinking of using Eldar as the primary army and using 2 detachments of Dark Eldar (One Coven and one Cult)

No, but you could do it if you took the DE as two Combined Arms Detachments (CAD) and the Eldar as an Allied Detachment.

Basically you can have as many CADs as you want now, subject to the minimum 1 HQ and 2 Troops for each. One of these will be your Primary and that will be the one from which your Warlord is chosen. You can also take an Allied Detachment but this must be from a different faction to your CAD.

All these new terms threw me for a day or two but basically you can now take multiple Force Orgs in the one army (say 4 Archons, 4 Warrior units, 6 Ravagers, etc is now legal) but allies use the same 6th ed allies restrictions.

you can take any number of detachments, so you can take multiple allied detachments in battleforged