The format page for 0.15 inch wide tape has a drawing (click for large version) that clearly shows that mono cassettes have one wide track and stereo cassettes split this track in half and add a small guard band. Most mono cassette recorders follow this format. It turns out that the mono Marantz PMD201 uses a two-channel head and records dual mono. Most other mono recorders seem to follow the standard.
While this is a theoretical problem, few if any good mono recorders are available for reproducing these tapes anyway, so most of us in the domain transfer field use good quality stereo machines for all cassette transfer work.
UPDATE 2012: While it is often possible to sum the channels if the Dragon finds proper azimuth, on many low-quality tapes this proper azimuth is not achieved. Now that azimuth correction (at least the time delay portion) is available for a reasonable price in iZotope RX2 Advanced, I have found that after correcting for time delay between channels that I can now sum both (if they are worthy of it) and reduce the noise prior to applying the noise reduction plug-in.
This is an interesting anomaly that should not effect operations. If you had a full-width mono head, there would be at most about a 4 dB improvement in signal-to-noise ratio vs. using one track of a stereo head (which usually results in better sound). Since the premium cassette decks, such as the Nakamichi Dragons that I use, provide very low wow and flutter, flat frequency response, a means to adjust azimuth of the play head, and gentle tape handling, all of these benefits outweigh the 4 dB or less reduction in signal-to-noise performance. If the tape hiss is a real problem (as opposed to noise recorded in from the recorder’s electronics), some of it can be effectively removed using a noise-reduction plug-in. Generally a 4 dB noise reduction in a good plug-in is transparent.
We highly recommend archiving both tracks in a professional archival scenario.