The layout and properties of the foot note separator are a document-wide setting. It does not depend on the chapter, section, even/odd page etc.
There are two remedies:
- Choose different Word documents (files) for each separator type
-or- - Use fields to create a mock-up version of the line: Delete the line, insert a field as described by Microsoft. You didn't specify where you want which type, so I cannot help yet with formulating the condition in the IF field, but the values should be something like "--- " and " ---". Of course you would use more spaces and a hyphen that doesn't leave gaps when repeated. Use Insert -> Symbol or the Windows Charmap program to find candidates.