Having 2 different files with 2 different languages in app_code folder:::::::::::::::::::::::::::

put all CS files in CS folder under app_code
put all VB files in VB folder under app_code
and do put the following in web.config file

<_compilation _debug="_">
<_codesubdirectories>
<_add _directoryname="_">
<_add _directoryname="_">
<_/_codesubdirectories>
<_/_compilation>