Civilan,
The Tornado uses the combined Y-axis optic + encoder board from the Happ trackball -- at least, it was originally designed to do that, before Happ changed the layout of the main board, moving the Y axis optics to the center of the board instead of the edge (which ruined the layout for the Tornado.)
The current Tornado uses both Happ optic and encoder boards, (X-axis optic board connected to the Tornado, then connected to the Y-axis optic + encoder combo board via a cable) but that is due to change, and SlikStik will be going back to a single optic/encoder board again as was originally designed.
In the meantime, if you get a Tornado, you could actually hook up two spinners to the electronics, since you have both optic boards (X & Y axis) from the Happ trackball. You could even remove the Y axis + encoder combo board and use an Optipac, but there's no need to since you've alreay got an encoder.
Be warned that the dual-optic boards on the Tornado is a "feature" that will be removed when the board design changes.
More pics of the current two board setup can be found in my full review of the Tornado:
http://users.adelphia.net/~kevsteele/mame/tornado.html*Note that the first picture of the Tornado in the review is a picture of the prototype, and shows the original single-optic/encoder combo board being used. The other pictures show the dual-board setup, and if you look closely at the second (unattached) board, you can see the change Happ made, moving the optics to the middle of the board instead of the edge.
Kevin