Sonars tend to have wider beams, on the order of 30-40 degrees, while GP2 devices
tend to have much narrower beams, 5-degrees or so. You probably want a wider
beam device to make the initial detection, and possibly a narrow beam device to
OTOH, I keep thinking I could do it all with 2 wide-beam sonars [40-deg each], and mount
them so as to have a small degree of overlap on the center edge of the beams. That way,
only one sonar would pick up an object moving in from the periphery, and would trigger
the servo to move them until both picked up the moving object simultaneously. Then, as
the object moved around, it would move out of one of the beams, and cause the servo
to move to track it.
I keep thinking this might work, one day I'll try it. :-).
You might also checkout Rod Brooks' old robot Genghis. It used multiple PIR sensors to
track and follow after moving persons.http://www.ai.mit.edu/projects/genghis/genghis.html