How to Assign 2 roles for 1 user

Hi guys,
Im now in assigning a roles to a user. But, 1 of my user wants me to assign 2 roles for him.. The situasion is like this :
I have a few roles which are :
- zone head
-zone officer
-clerk
The problem is now 1 person can be assign for 1 role only..
How can i assign 2 roles for 1 user.. ?
 
anyone have experienced these before..?
Thanx
...