Basically the welsh FA will sell tickets for both sometime in the new year, you’ll get tickets for both games but if wales fail to win the semi final, the second game will be a friendly against the loser of the Poland V Estonia game. If wales win the semi then the second game will be the final against the winner of Poland V Estonia. You can wait to be sure if they win or lose but you risk missing out on tickets. The same happened at the World Cup qualifiers, we(wales) got to the final and played Ukraine. If we had lost the game before we would have played Scotland who lost against Ukraine in their playoff if I remember correctly.
Basically the welsh FA will sell tickets for both sometime in the new year, you’ll get tickets for both games but if wales fail to win the semi final, the second game will be a friendly against the loser of the Poland V Estonia game. If wales win the semi then the second game will be the final against the winner of Poland V Estonia. You can wait to be sure if they win or lose but you risk missing out on tickets. The same happened at the World Cup qualifiers, we(wales) got to the final and played Ukraine. If we had lost the game before we would have played Scotland who lost against Ukraine in their playoff if I remember correctly.