Hello Dmeyster,
Did you check
this article?
1. Both players should turn 8th skills somewhere in the game lobby before joining/creating the game. You can do that in Multiplayer -> hotseat or singlplayer -> custom game
2. You should do that every time the Heroes V exe file is started and before loading/creating a game with 8th skills. This should be done because the save game does not keep track of the number of allowed skills.
Let me know if you have any questions.