Character with Two Classes

Players Handbook (page 33)

Unlike multi-classed characters who are of non- or semi-human race, the character with two classes must be human. To attain the second class, the character must switch his or her profession at some point. Thereafter no progression in the original class is possible.

House Rule

Any race can switch to another class after level 5. But they become the other class at level 1, though they retain the hit points and skills of their former class (as long as they keep practicing the knowledge they have gained).