Actually you can use archetypes even if they do overlap, benefits to the same ability do not stack. So if archetype one grants a +5 to perception and archetype two also adds a +5, you only get a +5.
Or at least this is how I understand it from reading the book. "Archetypes are a quick and easy way to specialize characters
of a given class, adding fun and flavorful new abilities
to already established adventurers. The class archetypes
and corresponding new abilities mentioned below are all
included in this chapter, and characters may take more
than one archetype as long as they meet the requirements." Ultimate Combat page 22.
The problem is when the archetypes change the same thing or rely on the same swap off of special abilities.