It used to be the opposite, that 2 fares were more than twice as expensive as one.

Example: Flight has 1 P fare but 2 F fares. If a couple booked on the same reservation, they would both get F fares, not 1 P and 1 F. I guess now the P fare for 1 is more than the F fares for 2.

This was partially mentioned above, but was worth emphasizing.