"Only one faction per realm on PvP realms" and other just announced limits

[Assami]

You can play your horde team on a different realms.
It won't change anything since it's not the same faction so play your alliance teams on one and your horde teams on another one

[Moorea]

The reason for having both on the same realm is to transfer money and/or items through the neutral AH

Also handy if someone ninja loots you and you can come and camp them from the other faction, etc...