J.-
I agree 100% with Sarah. He is looking for attention. Better for him to get it for good behavior than for bad. Make sure you are talking more to him and giving him more attention when he is acting good than when you are monitoring him during/after he's been bad.
In regard to the scenario:
Charlie :"I will knock Joey down and sit on him"
Me: "do not do that Charlie or you will get a timeout and Joey will be sad"
Charlie - knocks Joey down and sits on him, and get's a timeout.
How about:
Charlie: "I will knock Joey down and sit on him."
Mom: "That would make both Mommy and Joey sad. That's not how a good big brother acts. What are some ways that you can be a good big brother?"
And if no comments are forthcoming, maybe suggest a few - verbal encouragement from big brother in Joey's attempts to walk, singing a song for Joey while he tries to walk, etc - and maybe these are things Mommy can participate in too along with Charlie to make him feel like a "grownup" just like Mommy, and not like a baby like Joey.
I only have one child now, but I've read that it's best to address things to the older sibling from the older sibling's point of view. For example, don't say "Baby's eyes are so blue, aren't they pretty?" Rather say, "Baby's eyes are blue, just like yours!" Also, not, "Baby is trying to walk, leave him alone," but "Baby is trying to walk. You did that too once, when you were a baby. But you're a big brother now. Can you show him how a big brother walks?" Or whatever :-)
Good luck! :-) Two is a hard age, you'll get though it!
T.