Not quite. 2:1 is 2x magnification, ie double lifesize on the sensor. 1:2 is 1/2x, ie half lifesize.
Your lens and most true macro lenses (except the MP-E 65 which is a special lens) can only go to 1:1 max magnification as a bare lens. This is at their minimum focus distance.
You can add extension tubes between the lens and camera body, and what this does is allow the lens to focus closer than it's min focus distance as a bare lens. This will give you greater than 1:1 magnification. I know, because I did testing/measurements, that your 100mm lens with 68mm of extension tubes (the full Kenko set) will give a magnification of 2:1 at its new min focus distance.
Note that with extension tubes you lose infinity focus. So although you can focus closer, you probably couldn't focus on something say 3m or more away.
This shot shows my first macro setup from years ago. The 100mm lens you have, extension tubes, and a very embarrassing milk container diffuser:
http://photography-on-the.net/forum/...13#post4426813
But the milk jug worked great - nice big light source for diffuse light and got it down over the lens with and without tubes.