There are quite a few combinations that don't work together. Some depend on class, some on race. If you look on page 49 of your manual you will see a list of Multi-classes. You can Dual any class that can Multi, depending on Race. Another thing you cannot do, is Dual to the Kit type of a class. You can START a character as a KIT type, though, and then Dual to another class WITHOUT a kit.
That sounds a bit confused, so:
Start with HUMAN SHAPESHIFER Druid then Dual to FIGHER (No kit)
Start with HUMAN BERSERKER Fighter then Dual to DRUID (No kit)
Start with HUMAN FIGHTER (NO kit) then Dual to DRUID (NO kit)
Only your starter character gets to have a kit.
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