Stereo = repmat(Mono(:), 1, 2);
In the case where Mono is already known to be a column vector then this can be simplified to
Stereo = Mono(:, [1 1]);
However, you will not be able to separate channels back to their original -- not unless there is some distinguishing characteristic of the two. For example if the original two channels were sine tones of different frequencies, then Yes, you might be able to separate them out. Two singers that are not harmonizing, and which have distinct voices... might be doable with some work. Left and right click tracks at irregular times... NO.
