Merge Audio Layers

Question

Hey Guys... I was just wondering if anyone had any idea how to merge to sound from 2 (or more) audio files into one file? I do not mean merge the files into one big one by concatenating audio2 to the end of audio1, however actually merging the sound layers over the top of each other so the final result is both audio files playing together. E.g. 2 track sof the same length, Guitar track + Bass track = One audio track with BOTH guitar and bass. Any suggestiong would be splendid. I have googled for a while, maybe im getting the wording wrong because 'merge' suggests appending the files, which is not my desired result :) Thank people, Chris Rickard.