Well the main reason to mule is being able to play twice. Many quests, hidden mickeys, pdf prizes, etc. can only be played once per character. So if you want more than just the one prize you can play again using another character.
Many people also like to have both a girl and a boy, so they can get all the costumes etc., because some are only for girls or only for boys. You can also do a credit run on each of your characters to earn easy money.
To have your characters meet on vmk you need to have 2 playing windows open at the same time. Just axcess the internet how ever you get on (broadband, dial up cable, LAN, what ever.) Then open 2 windows most people like explorer or firefox for vmk trading. Most windows based PC's all ready have explorer on them so shouldn't have to download anything. Axcess the vmk log in page and log in to each character. (Use the direct web address to login it works better than going to vmk.com. It's on the guide at the top of the vmk board but I will go ahead and put the link at bottom.) It works best if you log in to the first one wait till its finished loading, then do the second. Once the first person is logged in go into any room. Look at the
I (on the bottom left of the control bar at bottom) it will have what instance of the room that you are in. (I.E. Main Street - North East South) Once your second character is logged in click advanced mode on the map. Use it to select the same instance of the room that your other character is in. Have one of your characters ask to be friends with the other. That way you won't look at what instance the other character is in the next time. just go to your friends list and click enter same room. Trading is easy it works just like trading with other peoples charters only you are controlling both characters. (so don't make a bad trade with your self! lol...)
Have fun and happy VMKing.
https://secure.disney.go.com/vmk/en_US/index?name=LoginPage