Stereo cameras have perfectly parallel lenses. True, our eyes may "toe in" as we call it but as a rule, all stereo cameras over the last 100 years or so have had non adjusting parallel lenses. I tried parenting the aux camera to the main camera, copy and pasted the keyframes from the main to the aux then moved the aux camera's key frames 3 blocks to the right and I think that may do it. I have some more experimenting to do.

 

I'm attaching a stereo Poser image. If you cross your eyes, you'll see 3 images. The middle one will be in 3D.

Mike