I want to prune my 2 green ti plant so they will both grow 2 or more stalks. 
Exampe of how I want it to look: 
http://www.tiplant.com/tiplants.htm

My plants currently has one single stalk. One plant is about 1 foot to
where the leaves grow out from the top of the stalk. The other plant
about 2 feet. The plants are growing in 12" pots.

I am looking for specific directions on how and where to cut (credible
source since I don't want to damage the plants).
How big or old does the plant need to be? 
I have a couple Ti Plants in 4-6" pots and wondering how big these
plants need to be to prune.