To create a plane bisecting the supplementary angle of the default solution, select the Reverse feature 
button of one of the reference planes. To invert the normal of the new plane, select the Reverse feature buttons of both reference planes.
In this example, the vector of P2 is added to the vector of P1 to create a new plane with a vector of P3.
Click the Reverse button of one of the reference planes to create a plane that bisects the supplementary angle. The following example shows the new plane that is created when the normal of reference plane P1 is reversed:
