We have approximately 15000-20000 active users who contribute in topics on a daily basis. It would be really convenient for them to have the option of spotting whether another user is already active/typing in the same topic. Especially handy for our Super Users who try and help as much as possible. Sometimes we'll type an answer even though someone else has already posted theirs, but we weren't able to see that the topic was already being processed by someone else.
Related to this idea: Ability to see who else is working on a question or looking at a question in the backend, or see when someone is typing in the frontend | inSpired (insided.com)