Hi there Guy
You're right people do usually seem to have only one true phobia at a time. Although, as you say they can have multiple fears.
Essentially the same phobia can manifest it self in many different ways. So, for example, claustrophobia can present it self as fear of flying, cars, small rooms, lifts etc but is essentially the same phobia; pattern matching to lots of different triggers.
For the same reason that, usually, we only have one deep loving (romantic) relationship at any one time, the emotional centres of the brain respond to a very specific pattern to make us feel a certain way and it has to be selective.
It makes survival sense to be very frightened of specific things (or things that 'remind' one of the phobic stimuli) but not too many things otherwise we lose the ability to hunt, gather food etc. In the same way if we felt deeply in love with 2,3,4 or more people life becomes difficult to manage!
The Amigdalas (the parts that processes emotional response) are actually quite small so perhaps they can't manage too many fears at any given time.
Mark.