If you already have YM installed and you want to chat to more than one person at the same time, use the conference option in Preferences. This mean dthat the 3 or more online can chat to each other and also se their webcam. Its quite handy for families split around teh world!!!
If you want to have to versions of YM open.... its not possible. Only one account on a computer is possible to have open.
You can always do what I do.... run 3 computers side by side!!!
But I make websites and other media presentation so I need them!!!