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.
[fixed typo]