As I recall, this is not possible. I remember trying to set this up when I installed my first Win2K AD server. I was going to create a different domain for each sub company and have them all on one server. Each time I attempted to setup the new domain, I lost the previous one. You will need to have additional servers for each domin. You can then group them into a forest.
Hope this helps! Be sure to check the MSKB articles on this one just to be sure.