Is it a way to get know if more than one user edit the same node?
This is already built-in in Drupal 6 (and 7), in the function node_validate.

The relevant piece of code is reproduced below:

if (isset($node-&gt;nid) &amp;&amp; (node_last_changed($node-&gt;nid) &gt; $node-&gt;changed)) {
  form_set_error('changed', t('This content has been modified by another user, changes cannot be saved.'));
}


It doesn't lock anything, but it shows the second editor a message, and prevents the second editor from saving and thereby overwriting what the first has done.

The second editor can then copy-paste his/her efforts to Notepad or other temporary buffer, reload a fresh version of the node (i.e. a version that contains the first editor edits), add his stuff again, and save.