Two ways to do this...

One, spend 30K on high value items you know you can resell on the horde side.... list them for 1c each on the neutral AH and buy them with your other account... do this 1 item at a time. (or 1 stack). You stand a risk this way of losing high value items to people who snipe at the auctions...

The second is to list a gray from the horde toon for 30K gold on the neutral AH. Then buy it with the alliance toon. You'll lose 15% or something absurd in a border crossing tax... but theres no way you can lose on this one. the ONLY way this can go wrong is if some dipshit buys your item for 30K... and since it's a worthless item, if they do, you now have 60K, 30 on each side of the fence, which isn't really "going wrong"....