If a shortcut doesn't work as expected (i.e. as mentioned in the help text/manual) check whether you configured the same shortcut as global shortcut in System Preferences > Keyboard > Shortcuts. If you set up a "concurrent" shortcut it may also occur in Services, Screenshot etc.
Also check the app's preferences whether you set up custom shortcuts which override the app's default shortcuts.
It's not necessary to set up a custom shortcut for Word's Paint Format tool double click.
The logic behind the Paint Format tool button compared to the default shortcuts works like this:
- Choosing some formatted text and clicking Paint Format tool (1st click) equals cmdshiftC (copy format)
- Choosing another part of text and clicking Paint Format tool (2nd click) equals cmdshiftV (paste format) - the Paint Format tool button is resetted afterwards!
- Choosing some formatted text and double-clicking Paint Format tool and formatting several other text parts afterwards equals cmdshiftC and several cmdshiftV.
If you copy a text format/style with cmdshiftC it will remain in the "format pasteboard" until you copy another text format/style or quit Word.