Yes, a nearly empty colon will easily hold two quarts of water. If you can't hold all of it, just clamp off the tube and void the water that's in you and then start again. Enemas are really quite easy to do as long as you don't try and overdo things or tamper with the equipment. Some people try the douche tip for enemas and that is not good and could risk a puncture or bruising of the intestinal wall.
I have never used Colosan (too expensive!) but I do use its primary ingredient Magnesium Oxide in tablet form. It works very well for me to keep things going after flushing. The main thing is to be sure and stay well hydrated. I find water with a little lemon juice added works much better than water alone. The Vit. C also helps the oxygenation process of the mag. oxide.