If you cut the stereo file in half, check the peak level on the first half and then normalize. calculate the diference between the origial level and the normalized level and use the "leveler" plugin to boost the 2nd half by the same amount.
Another way of doing this: some s/w will calculate the gain required to bring up the entire recording to 0dB (or some user-defined threshold). So do this with the complete WAV file. Then, after splitting the stereo WAV in half, simply apply the same amount of gain - as calculated above - to both files.