Yes — Photos Backup Anywhere supports Apple Family Sharing
If the app is shared via Family Sharing, each family member can install and use it on their own Mac with their own iCloud Photos library. The app works with whichever Photos library is signed into macOS on that machine, so you can absolutely use it to back up your partner’s images as well.
If the app is shared via Family Sharing, each family member can install and use it on their own Mac with their own iCloud Photos library. The app works with whichever Photos library is signed into macOS on that machine, so you can absolutely use it to back up your partner’s images as well.