I have one of these:http://www.greenpowerscience.com/BOTTLECUTTING.php3
It has worked very well for me. The design is great because it rotates from the center rather than the edge. It accommodates a wide range of sizes.
A few tips:
The aluminum is a little bendy, so be gentle.
Keep the neck snug on the cone.
Go smoothly, go evenly, but never go back! Stopping short by a millimeter or two is better than overshooting your starting point. To make it easier to see, I mark a blob with Sharpie before beginning and start there.
Use a little oil on the wheel. I use 3-in-1 because it was lying around. You may want to check with stained glass artists to see what they use.
I think the video shows it being used upright. I am butterfingered, so I put it flat on a table.
Be sure the arms are perpendicular when the butterflies are tightened.
Use boiling water. Not hot, boiling. I got an electric kettle for $20 that plugs in right next to the sink. It also makes great tea.
Alternate boiling water with cold, running the streams right on the cut line. Watch for the mirrored ribbon to run around the bottle and listen for the clear bell-like tinkle. A dull sound indicates a crack or irregularity that will probably be fatal to one or both pieces.
Hope this helps. Happy bottlecrafting!
That video is pretty impressive. Have you been pretty successful with this method? As in more than 50% of the time, lol.