In the description it states that the Defensive awareness ability and the Uncanny dodge ability are cumulative (If the Defender has a class that gives him uncanny dodge). How could these be cumulative?
The same way rogue uncanny dodge and barbarian uncanny dodge stack. The simple way to handle it is that you add up the class-levels of the all the classes that grant it and then look at how far it would have progressed for a single class character of that level who'd have the ability.
Another way of putting it is that Defensive Awareness is just Uncanny Dodge under a different name.
The text says: "If the dwarven defender has another class that grants the uncanny dodge ability, add together all the class levels of the classes that grant these two abilities and determine the character’s defensive awareness ability on that basis."
Assume you have a multiclass Rogue 1, Dwarven Defender 5. Add the levels to get 6. The character has the defensive ability of a 6th level defender (or rogue), which means he cannot be flanked.