Since someone wants me to post it again...
1) Take both your horde and ally characters to the neutral auction house in Gadgetzan or Booty Bay.
2) Sell some useless item for X amount of money.
3) Buy with your other toon. (The rich one)
4) Poor toon makes 0.7X due to AH share of transaction (where does this money go...)
5) If you don't like the post, STFU
** A more efficent method, as pointed out by Mordo:
1) Sell valuable items (primals nethers, etc.) to your poor toon for 1 copper via neutral AH
2) Sell these in the regular AH for a regular price
3) /dance the night away