We had the same problem with our daughter (who is now nine). We used to make the same Jekel/Hide analogy. She usually only hit me if I tried to put her into a time out. She would never hit her father, just me. We did not have problems with her hitting other children at preschool, although she does hit her younger brother...
I've read MANY parenting books and have seen psycologists about it. The level of violence escalated from just hitting to hysterical, hyperventalating, hitting, screaming, biting, and scratching... These violent meltdowns were almost always triggered by my trying to put her in time out, even though she knew we used a timer and it would be relatively short (we used the one minute per year guideline.)
One psycologist said that some children can't handle time-outs because they see it as a rejection and feel abandoned. They suggested sitting quietly with the child and having a time-out together or giving up on the time-out approach altogether.
I'd recommend books called "1-2-3 Magic" and "Raising Your Spirited Child," and if things continue to get worse, "The Explosive Child".
Honestly, we never did find a way to handle this issue that worked for our family. Especially when you have more than one child fighting, you really need to be able to separate them and send them to their rooms. At nine, our daughter now only has violent meltdowns once in a while, but she still really hates to be sent to her room.
Good luck!
-D.