I don't think so.Can Time Capsule be used to back up my Windows 8.1 side?
Maybe not so much through time machine but just as a disk I can see when in windows. Im using it with 8.1 with Boot Camp
According to Apple it is Windows compatible so I don't see why not.
http://www.apple.com/uk/airport-time-capsule/specs/
If it was a Windows PC network then perhaps to the TC. Windows backup needs an NTFS drive to backup, the TC of the OP will be formatted in HFS+ for TM backups so a winclone image to the OSX boot drive is the easiest option for TC to automatically backup.
Couldn't he create a partition for each?
I don't run windows on my Mac so I could be totally wrong of course.