i dont really know of a good way to go from a stereo track to dual mono. if you have multiple clips, then the bus option you did is probably best. if its a single clip, you can double click it to enter the waveform editor, and from the menu choose Edit > Extract channels to mono files. you would then have to bring those new files into the multitrack manually.
audition is a very powerful tool, it has features that could cost hundreds if not near a thousand dollars... however the multitrack functionality as a DAW is very lacking. adobe is not an audio company and as such when they bought this software (formerly cooledit), they did very little to keep it up to date with the rest of the industry. there are workarounds for alot of things, but they start adding up to wasted time instead of using a modern fully featured product that can do all these tasks.