Allies?

[Pater Sin]

If I use my Tau as my primary detachment, can I have both DE and IG as allies since they are battle brothers with Tau or do I have to pick just one of them.

[Mr Mystery]

You can only have a single allied detachment.

[Charistoph]

You can only have a single allied detachment.

Even when doubling your FOC, you may only use the same Ally army as the first Allied Detachment.

Anything else is House Rules for your local group.

[rogue.trader.voril]

Pg. 110 ) At 2000 points you can split to 2 primary detachments, allowing two allies... which both have to be out of the same codex. Short version: No, unless you are playing Apocalypse.

Dark Eldar (DE) are not battle brothers with either IG or Tau, but Eldar are.

*Beaten to the punch again...