You'll need, 2x Characters, Partied & money for a Faction Change.
How To:
1. Get 2x Hordes/Alliance Characters.
2. Initiate a faction change on one of the characters, while being groupped.
-> You must be at the "starting" location for whichever faction you choose(The place you'll start whenever you factionchange).
3. Wait for the fac. Change.
4. Re-log, a few times(logging into your account, not into the character) - Do this on both characters.
5. Your horde char should now be in group with your alliance char, and you'll be able to interact, talk, duel, trade and so on.
6. Once you quit the party, this no longer works, and you cannot ever interact in the same way with that character.
Enjoy.