subfolder web.config ..

Hi all,
I need to call or link two .config files.
consider..main config file web.config is in root directory.
and another a1.config is also in root directory
and another b1.config in sub directory(i.e root/TestFolder/b1.config)
now i need to link or call a1.config and b1.config from web.config.. how to do it
the a1 and b1 config also has "configuration" main tag in it.
solution plz.. Thanks in advance.
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