Think about your closest friends. If you know them well enough, you also know their strengths and weaknesses. You know how they think, how they react, how they tick. You trust them to know the same about you, too. You might even trust them enough to let them fix your car--but not drive it. Hell would freeze over before you let them drive it. It is not uncommon in certain stories to have two separate badasses who come together, spend some time trying to kill each other only to eventually turn their skills onto the hapless enemy who happens to piss them both off.