While booking a flight today, the “best available” fare offered by the American Airlines web site was higher when booking for two passengers rather than one. It appeared that there was only one seat available at the lower fare, so the site bumped both seats up to the next highest price. Nice.
In this case the difference was about $100, so it was worth it to book separately so that at least one of the tickets was at the lower price. So, it would seem that it pays to compare the single passenger price to the multiple passenger price before booking.