I think I would first try just flipping left and right in the call to glFrustum. The only errors lists are near or far not being positive or trying to call glFrustum in the middle of a glBegin/glEnd block. I also think I would use two viewports for testing. That should make it easier to figure out just what went wrong. I tend to code by successive approximation so I would first test that flipping left and right worked as expected, then that I reflected the position and view angle correctly, then that I clipped it correctly and finally that I merged them correctly. I''m basically lazy so rather than spending a bunch of time walking through the code I let the computer tell me what the code does.