Edit the VirtualKeys and VirtualDirectory elements in the Vhost.xml file as follows:

<VirtualKeys>
    <Key from="WIN 7,0,19,0" to="WIN 9,0,115,0">A</Key>
    <Key from="WIN 6,0,0,0" to="WIN 7,0,18,0">B</Key>
    <Key from="MAC 6,0,0,0" to="MAC 7,0,55,0">B</Key>
</VirtualKeys>
<VirtualDirectory>
    <Streams key="A">foo;c:\streams\on2</Streams>
    <Streams key="B">foo;c:\streams\sorenson</Streams>
    <Streams key="">foo;c:\streams</Streams>
</VirtualDirectory>

For more information about editing the Key and Streams elements, see Adobe Flash Media Server Configuration and Administration Guide.