The Gold Coin Merger denotes the joining of House Wu Tzun and House Medina after the Second Succession War. The newly founded House Jehanni flies a golden coin as their flag, giving the name to this event.
Longtime friends and occasional favourite enemies, House Wu Tzun and the Medina were both less powerful in military terms then either the Houses Habichtswald, Dracon or Andragor, but both rather mercantile and filthy rich. Both Houses valued education and spirituality, but while House Wu Tzun emphasised austerity, the Medina reveled in savouring and showing what they had.
With the beginning of SW II, both Houses refrained from allying themselves with either the Habichtswald or the Dracon. Instead, they entered a pact of mutual assistance, hoping that the other Houses would weaken each other enough to become easy pickings for them in the end.
Unfortunately, the Medina holding of Agrabah became one of the most heavily embattled systems of the war, as it is a connecting hub for several of the most important jumproutes, especially the shortest connection between the Dracon worlds and P2. Both the Medina and the Wu Tzun suffered heavy losses in defence of the system, reducing their numbers and assets to the point that they couldn’t have possibly survived as Royal Houses.
Faced with the prospect of insignificance or extinction, House Wu Tzun and the Medina reached an unprecedented agreement - they would merge as equal partners and become a new Royal House, House Jehanni. As peace offering to the new Emperor, they sacrificed several of their less valuable planets, thus ensuring both the benevolence of the Emperor and their own viability as a Royal House by concentrating their resources.
So while technically not extinct, they no longer exist as separate Houses. Even to this day, the different mentalities of the Wu Tzun and the Medina can explain many of the internal struggles of the Jehanni.