I don't know what you mean by "Yahoo for web?" You're probably using Firefox or Internet Explorer as your browser. You can have/download both Yahoo! Messenger and MSN Messenger. You can put the same contacts on both, but to chat to them on MSN, you use MSN Messenger. But they would have to have MSN Messenger too. The same goes for chatting with them on Yahoo! Messenger... they would have to have that chat program. I don't use MSN Messenger, but I do use AIM and Yahoo! Messenger, and chat with some of the same people on both.