We're really closer than you might think! When the sunlight is more on one than the other, the motor moves in one direction or the other.
When they are equal or almost equal, both Q3 and Q4 are OFF, leaving R1 and R2 to weakly pull up the gates of Q1 ad Q2 to 12V. This is what keeps Q1 and Q2 OFF. XMM1 and XMM2 show 11+ volts, about equal, indicating no power on the motor.
The only state that needs work is when there is no light. When both inputs are 0, the op amps still get equal voltage (0) and don't know what to do.