There are several methods to extend a single user's desktop to span multiple monitors (or clone between monitors) 

* [[RandR|RandR]] 
* [[Xinerama|Xinerama]] 
See [[http://mulchman.org/blog/?tag=displaylink|http://mulchman.org/blog/?tag=displaylink]] for a good general post on setting up ExtendedDesktop on [[DisplayLink|DisplayLink]] devices