Ultrasonic / Sonar Rangefinder trigger / turning angle

Hi I have been looking at trying to set up sonar for the rover I am putting together.

Based on these links:
http://rover.ardupilot.com/wiki/sonar-sensors/

http://rover.ardupilot.com/wiki/apmrover2-parameters/#Rangefinder_trigger_angle_APMrover2RNGFND_TURN_ANGL

The description in the first link indicates that by using two sensors you can get the rover to turn in the opposite direction of the obstacle. However to do this you would have to have two different trigger angles but I can only find one in the Parameters list?

Is this correct am I missing something?

Any good discussions I may have missed on implementing sonars?

Thanks in advance

@obbila,
The rangefinder function is setup with just one turn angle because the rover will turn in the opposite direction of the senor that was triggered by an obstacle.
Ex: Left rangefinder detects a target so the rover turns to the right a programmed number of degrees (TURN_ANGL). Right rangefinder detects a target so the rover turns to the left the same programmed number of degrees.
Regards,
TCIII AVD