Usually the Time Capsule's storage drive is formatted for use with OS X.
Since the older Time Capsule (Time Machine backups) are over-written
by the software as they go outdated or when storage space is lacking, a
Windows file backup on the same unit would not be a good idea.
If the 'PC' only implies a desktop computer running OS X, then it would
be able to make backups of the other Mac via Time Machine software
and the total capacity of the Time Capsule device would be lessened by
the addition of this other machine's demands for backup time capacity.
How large is the Time Capsule storage? I'd look to a separate device for
backup of a Windows system; however if a Mac is running Windows, it
may be possible to configure something to backup its OS, to other drives.
Here's an older but good source for some information about the OS X
and Time Machine (plus time capsule)
• Apple OS X and Time Machine Tips:
http://pondini.org/OSX/Home.html
Perhaps someone who has configured a workaround for Windows backup
(separate drive, perhaps plugged into Time Capsule/AirPort extreme?)
may reply; as there are a tremendously helpful contributors in this area.
Good luck & happy computing! 🙂