Two people, one board, and no document in the middle.
Digiboard is a collaborative whiteboard. Draw on one screen, and it shows up on every other screen in the room.
Two browsers, one board. The two cursors are two people drawing at once.
The idea it taught me
There is no shared document and no conflict-resolution algorithm. Every stroke is an event, and the server stamps each one with the moment it arrived. Every screen keeps the strokes it has seen and, whenever one changes, sorts them by that stamp and repaints. Conflict resolution is a sort. Two people drawing at once can never disagree about the result.