adding an amp to power new speakers on top of the stock bose amp

i have a 99(old body style)se pathfinder.i wanna add an amp to power new speakers going in the stock locations.i wanna know where i should spice in the speaker wire so that i can still use the stock amp on top of the after market one.what will i need(adapters,harnesses)i am keeping the stock bose head unit.i want the stock amp to power the a pillar tweeters but have the aftermarket amp power the door speakers front and rear.i kno lots o people have doen numerous stereo upgrades on their pathfinders just need a little help.wanted to know if anyone has ever run this setup before.thanks