First you need to calculate the matrix M, which represents the transformation from the world frame to the eye frame. For stereo, there is a left eye coordinate system which is different from the center eye coordinate system.
Lets do the calculation using the left eye.
The steps are the same for the right eye. When doing stereo you only translate horizontally, so the t and b values of the frustum don't change. The only ones that change are l and r. Remember that you have to specify l and r in the left eye's coordinate system.