How do I know the users who are editing same page?

If two users are editing the same node, the changes made by the user who submits first will be honored. The other user will be informed upon submission the node has changed since they initiated editing.

A method to lock a node and inform other users attempting to edit could be devised, but this creates some challenges. For instance what happens when a user walks away from their computer while editing a node? I suppose you could look at the user's session data and unlock the node after a period of inactivity.

Views 2 uses a locking system which could probably be adapted for editing nodes, but this could potentially take considerable effort and testing.

It's an interesting idea though and might be fun to explore.

I became curious about the idea of "node locking" and discovered there's already a module called Checkout that does just that.