Debugging a page thats in a virtual directory

[Tom Pester]

This is the simplified structure of my website in IIS :

c:\Website
-------> Home (normal directory)
-------> Engine (*virtual directory* but not an application!)
I made the Engine (which contains code I use in many sites and thus is
centralised) not an application because I use application variables within
this directory.

This is how I set it up in Visual Studio.net 2003 :

Solution "Website"
-------> Project "Website" (points to c:\website)
-------> Project "Engine" (points to c:\Engine)

I have to add the project "Engine" because VS doesnt add the virtual
directory automaticaly.
(I added it with "add existing project from the web")

I set my start page to c:\website\default.aspx and started debuging.
Once I get into a page thats located into the virtual directory where I set
a breakpoint VS tells me that the source isnt available so I dont get to the
breakpoint.

Once I make the virtual directory an application the problem goes away but I
dont want to do this.

How can I get the full power of debuging without making the virtual
directory an application?
I think I am lacking some fundamental understaning about asp.net and
processes. Can someone point me in the right direction please?