Two users completing separate webforms on Drupal that create one activity on CiviCRM
You need a custom or temp table which holds user1_ID, user2_iD and activity_id 
http://book.civicrm.org/user/current/organising-your-data/custom-fields/

So when you are creating the first activity set the activity status to schedule.

Create a new activity Type for your process if needed - 

You need to create an extension and use hook_civicrm_post - set the activity to completed 
http://wiki.civicrm.org/confluence/display/CRMDOC/Create+an+Extension

If you need more help please let me know - thanks