Create auxiliary folders and put all your VB code in one folder and C # in another. In your case, you will have one for C # and another for VB
You will need to modify the web.config under the compilation element to include them so that the compiler knows to include them in the compilation
<codeSubDirectories>
<add directoryName="MyVBCode"/>
<add directoryName="MyCSharpCode"/>
</codeSubDirectories>
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