Using Prismatic Wedges
As shown in the simple ray diagram below, a clear plastic wedge with it thin edge down will deflect light rays upward. This positions the image at a lower position than the object being viewed. A second wedge positioned with its thin edge up will deflect rays downward, thus positioning the image to a higher position than the object.
When used with an image pair such as shown below, the effect is to lower the top image and raise the lower image. This must be done in a manner that will overlap the two images. Three images will be seen, but as with the crossed-eye method one focuses their attention on the center image.
Of the image pair shown above, the upper image is to be seen by the left eye and the lower image by the right eye. Since the upper image must be brought downward, the thin part of the wedge should be at the bottom. Of course the orientation for the wedge in front of the right eye should be opposite. Depending on the viewing distance, the images will probably be moved more than required to merge them properly(i.e., they will "overshoot" the desired location). This can be corrected by rotating the right eye wedge clockwise and the left eye wedge counterclockwise. This reduces the amount of deflection for each upper and lower image. The image for each eye will be shifted sideway. Fortunately each image is shifted in the same sideways direction and they will remain in vertical alignment.