When the one user receive CollectData request other should have the possibility to backup them and put in data instead. I've granted right to do this for another user.
There are several issues surround this approach
1. Another user cannot use Edit form on CollectData workflow task but can edit data using datasheet view. That's a bad news - the other one has the rights and even could be inserted in ther AssignedTo field.
2. Closing up the task by another user cause workflow error
What the correct way to implement the task when one user is a major responsible person and other backup one