Hi Vivek,
Check the below link, I think it would help you with the configuration.
One Web Dispatcher, Multi Systems with URL Prefixes
And also read the below document for the Mapping of one SAP Webdispatcher to Multiple Systems.
http://help.sap.com/saphelp_nw73ehp1/helpdata/en/b0/ebfa88e9164d26bdf1d21a7ef6fc25/content.htm
Regards,
Sushil