The way round this is to deliberately cross your eyes and get the two images to overlap, then if you continue to hold the overlap, they will snap into focus. it takes a bit of practice but it's easier than trying to do it without crossing.
The reason is that focus and parralax are linked. If you try the trick with a piece of cardboard on your nose, and two images either side of the board, you need parallex set to infinity (parallel view) and focus set for reading which is the absolute opposite to normal eye function. You *can* train yourself to do it, but it's hard.
In the crossed-eye version, you have focus for reading combined with an unnatural eye position that is compatible with close focus, but has no defined association in normal viewing, so you're not trying to override anything (other than the normal limits on eye-crossing!)
You may need to swap the images and put R on L, since you're looking at the "wrong" ones with each eye - tends to invert relief.
Space/Science » in reply to re: Viewer for computer screen: http://www.pokescope.com/
Seeing in Stereo