Stereo MPO files (such as those produced with the Fujifilm Finepix W3 camera) store the parallax value in a tag in the EXIF information of the second image in the MPO file, but it appears that Pillow can only read the EXIF information from the first image at present.

The tag can be manually read via exiftool as follows:

$ exiftool DSCF3195.MPO -mpimage2 -b > DSCF3195-r.jpg
$ exiftool  DSCF3195-r.jpg -Parallax
Parallax                        : 5.625

I have several sample MPO files available:
http://valen.darkstarsword.net/photos/stereo/misc/originals/

I would like to make use of this tag in my Stereo Cropping Tool ( https://github.com/DarkStarSword/StereoCropper ) to start with the correct parallax value.