Calling a web service from a web service is exactly same as Consuming the the web service in you application. There is no difference.
Let me explain in detail
1) Assume you have a "SampleApp" ( may be console, Winform, ASP.ENT app )
2) This "SampleApp" needs to consumes a web service ( say MyWSMain ), so the a web reference should be added to the "SampleApp" using "Add Web reference" and statred using its proxy class. Or instead of adding a web reference u could also create a proxy class of the web service using .NET tool "WSDL.exe" and it creates a proxy class (infact file with ur language of choic Vb.N#, C#, J#, C++) and u could use this file as good as a normal class file
3) Similary if MyWSMain web service wants to consume a web servcie "MyWSSub", then follow the steps as in Step2. No change in the way the web services are added and consumed . The consumption is irrespective of the application you use in