Time Machine performs best on it's Own Dedicated External Drive...
Mac 101 Time Machine
IMO... Better to get a seperate drive for the PC files.
That way all your eggs are not in the one basket.
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You can do it. Just create a second partition formatted on FAT with Disk Utility, so it will be read on Windows and you can copy files from Windows. The first partition must be formatted on Mac OS Extended (Journaled) to be used with Time Machine.
To set up Time Machine, open System Preferences > Time Machine > Select Disk and select the external disk. The initial backup will start automatically after setting it up