I have built a single axis version of the same thing, never used it just built it.
Its a pic16f88 microcontroller with 2 norp12 light sensistive resistors and a rc servo.
The micro positions the servo by adjusting a pwm o/p so that the light falling on the 2 cells produces an equal voltage within a bandgap.
Theres a couple of niceties, when the overall light level fails, the unit positions the solar panel facing east ready for sun up and goes into sleep, waking up every few secs to check the light level.
Shouldnt be that hard to increase to 2 axis.
I was going to build a r/c compatible servo from a car wiper motor, but never did for reasons below.
The problem with this method is clouds, the panel constantly positions itself in cloudy weather wasting precious power, I think systems that use gps and calculated solar position are superior.