If you're talking about the built-in Windows 7 Image Backup feature, it prefers to have a HDD all to itself, don't mix it with anything else, IMHO.
Windows 10's built-in "backup", is really just garbage, it saves "previous file versions" to your "backup" directory.
Get a real backup program, like Macrium Reflect (they have a free version, not just a free trial, although they have one of those too, make sure that you get the right one to use). Then set up scheduled backups, it can only do differential and full backups in the free version. (Although, one of the free versions tried to throw in incremental backups too, into the scheduled backup task, and then failed, saying unsupported. Watch out for that.)
Macrium can backup multiple systems to the same destination directory. (I use a NAS.) Just make sure that "Image Name" is unique per PC.