This system opens classes to explore others and add their skills to their own. They do so by expending their current experience points to purchase other class' skills. The experience points used to purchase skills are lost and cannot be used to level up the main class. Thus, players have to decide whether they want their xp to go towards their main class' progression or the secondary class.
By design, the secondary class will never outshine the character's main class nor outperform the standard class itself. You are not a Warrior and a Cleric, you are a Warrior with a little bit of Cleric (or vice versa). Multiclassing a Cleric requires a Main Class Level 3 before they can be as effective as a Level 1 Cleric, and a Main Class Level 10 with all Cleric skills purchased will be roughly equivalent to a level 6 Cleric.
These character sheets with details are provided so players can easily incorporate these into their own sheets.