Saving an XML file at the external application

Hi All,
I'm trying to figure out how to save my XML file in the outside location.
I have a Main web app that uses an XML file as a datasource for one of it's pages. This XML file is generated manually by accessing a utuility page in the same app. I created a utility app that  resides outside of the Main app ( it's on my C:/Inetpub/wwwroot) but it should save an XML file in the directory of the Main applicatoin. My utility works fine if I need to save to C:/Inetpub/wwwroot/XMLFiles) but I when I try to access the file at the Main application something like that:
objXML.Save(Server.MapPath("../../_MainApp/App_Dev/Marketing/Tests.xml")); 
It gives me an error: System.Web.HttpException: Cannot use a leading .. to exit above the top directory.  If I just type full URL it says that URI cannot be resolved..
How can I access that Tests.xml file to update it?
Th ...