Some characters may wish to play as a hybrid of two separate classes, and instead of sacrificing levels between two classes they may wish to play as one single class through their whole progression. This can lead to potentially stronger and more well-rounded characters, however for smaller groups (for example, a two character party) it could shore up a parties' weaknesses and flaws.
Characters select their first class and second classes, combining them together in terms of class abilities. Characters with spellcasting still gain MP, and characters with scaling bonuses (such as an Evoker's Blast or an Infected Host's unarmed) still receive these.
In regards to the HP & HP Progression, Skill Ranks, Soak, and Attacks per Round they take the better of the two between each class.
For example, Robby wants to make a Crusader/Rogue and thus takes the Crusader's attributes save for Skill Ranks which they instead use the Rogue's.
Some GMs may wish to take this a step further, allowing for characters to receive a free Advanced Class. When doing this, the Advanced Class becomes their Tertiary Class which they gain upon reaching 10th level (regardless of the Advanced Class' level caps and prerequisites).
The following below are example combinations and names given for the examples provided.
There are plenty more combinations with pros and cons among each class. Depending on which class combination you choose, you can unlock more potential to squeeze from your class - archetypes only adding to this capability.