Only through neutral auction houses.
The Soyuz goes to Treasure Bay or Tanales and places something worthless, sells it for a very high price, the amount of gold you sell your Tribe for, and then the Tribe buys it, and the money goes to the Soyuz.
However, the neutral auction house fee is high, 15%, that is, every 100G to lose 15G, but this is the only way, there is no choice.
World of Warcraft tbc Alliance and Horde cannot exchange coins or trade, only through neutral auction houses.