I am confused by this, but it's probably becasue I haven't had much time playing around with the newer builds.
Our heroes now can have dual classes? Sweet.
And I think we can bring up the advancement tree any time.
Where can you bring up the advancement tree? Your sovereign can not dual class, but some of the heroes you recruit have two classes listed, such as mage and assasin, but only one tree to choose from. At least as far as I can tell.