Multi-classing is the mixing of two classes together into one character. There are only five classes in Elemental D6, but there are still options for mixing these classes. By design, elementalists should only have one element. Whether or not you may mix elements is strictly up to the GM.

You can, however, always mix the martial class with any of the four elemental classes. When mixing two classes you would take a second class when leveling up. Your GM may require some event in your character's adventure's to unlock that skill if it is a second element. No such event is required to mix the martial class with another element.

When taking a level in another class, you take the level bonuses, skill bonuses, and talent points according to the level in your new class, starting at level one. You do not get additional bonuses associated with starting a character, your first level in a new class is treated as any other level in that regard.

Example
You have 4 levels in hydromancer and you level up to level 5. Instead of becoming a level 5 hydromancer, you put the level into the martial class. You gain 4 talent points for this level, as well as 3 + Int skill points, and no attribute bonuses (despite the level 5 attribute bonus for hydromancers).

Level Requirements
Talents often have level requirements in order to put your first point into the talent. For these requirements, you must have enough levels in that class in order to learn that talent. For all other requirements, including requirements for putting additional points into a talent, combine your levels from all your classes to calculate your character's level.