Not sure how to phrase this question but does anyone trying to talk to me when I am on line need to know I am already chatting to someone else? Hope you understand this garbled question. I am trying to avoid it being obvious that I am chatting elsewhere and not free to take the second request.

Couldn't you quickly press CTRL + TAB to change to a different conversation? Doesn't take a sec

There is no indicator that you are talking to someone else on messenger.

When you're talking to someone, then set a Personalized Status (you're supposed to have Messenger Plus! since you've posted this thread on Messenger Plus! Live Extension forum) and set your status Busy, and set an auto-reply like "I'm already chatting with another one" and filter contacts by selecting all your contacts except the contact you're currently chatting with. (Invert Selection button will help you for doing this) They will only be notified when they write to you. But you can simply state that in your personal message when you don't want to be disturbed.

Still, a script may do much more thing about this subject.

hmm that's a little troublesome since the number of conversations a person can handle varies, and it doesn't only depend on the number of conversations but also their intensity.