Pulse Width Modulation (PWM) can easily be achieved with two wires. All PWM does is hold one wire constant and rapidly pulse the other between 1k and 10k times a second. By changing the relation ship of how long the pulse is on versus how long the pulse is off the duty cycle is changed.
Both the wires from the pressure control solenoid go to the PCM. The probable operation of the solenoid is that there is a default position of the solenoid in the middle where the transmission is derivable. This position is held with springs so that if control is lost the solenoid will default there. This is there to provide a limp mode in case of electrical failure.
From here to raise the pressure one line is held at a constant polarity and the other is PWMed. This will cause the solenoid to move from the default position in the direction of raising the pressure. To lower the pressure the same thing happens but now with reversed polarity. This will cause the solenoid to move from the default position in the direction of lowering the pressure.