Please note that the original (and very convoluted!) answer is a work-around for an old version of Studio One, and the Bounce answer is for doing more creative things where processing of some sort may be needed. In version 4 and higher, to simply split a stereo file into two mono files, locate the file you wish to split in the Files panel (which you can show using F9), right-click the file, and choose the option listed. It is also just as easy to combine two mono tracks into a stereo one using the same method.