Using Startech.com 3 port hub with mini Display connector on XPS 13

Hi, I have a Dell XPS 13 laptop (just over a year old) for which I have a Startech mini display port hub (to display port connectors) which should allow me to connect up to 3 screens.   However, I have 2 Dell screens (new) and can only get them to show matching images.  I cannot get them to stretch the display across the 2 screens.  What am I doing wrong?  Many thanks, Angus.