I managed to make cross-faction guild and helped another person make one.
What the guild can do:
Both Horde and ally characters in the same guild.
Transfer items between horde and ally side with guild bank.
Transfer gold between horde and ally side with guild bank.
Both sides understand each others language, so they can chat to each others.
You could probably also sell these guilds for tons of gold, even at lvl 1.
Requirements:
2 accounts (works with you+a friend) - These accounts also need to lvl to 15.
3 accounts that help (not that hard to find people to help)
Step 1:
Make Panda and level it to 15.
Step 2:
Queue for Wailing Caverns, use /lfr to open dungeon window.
Requires 5 players to do this, you can find people in the Neutral Pandaren thread.
Step 3:
Die!
Ress at spirit healer in the cave.
Best to die again and ress at spirit healer at crossroads. (shorter walk)
Step 4:
Get to orgrimmar, buy a guild charter.
Step 5:
Get back to island (relog) and get signs for your charter.
One of these need to be your 2nd account or a friends account.
Step 6:
Choose Horde faction on 1 of the accounts that signed charter, Choose ally faction on the other.
Hand in the guild charter.
Now you have 1 character on horde side and 1 on ally side in the same guild.