You can't. Allies are not a thing anymore.
Your whole army must share a key word to be battle forged.
So Aeldari allows you to have Dark/Craftworld/Harlequins in the same army - or even the same detachment - legally and without any issues. The same can be done with the Imperium keyword which allows marines/guard/knights/assassins/etc to be in the same army and detachments.
But adding tau to an Eldar army? That's a nope in battle forged lists
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