Remove one directory from full directory path

I have a directory path of C:\Subdir1\Subdir2\Subdir3\filename.xml.

I need to first check if the file, filename.xml exists in the root of "Subdir2" (one level back) before looking in "Subdir3". How can I easly parse this with Path.GetPathRoot(filename)?

I.E.

First see if this is true:

C:\Subdir1\Subdir2\filename.xml

If not true, then default to C:\Subdir1\SubDir2\Subdir3\filename.xml for the file.