Messages in the Cloud with multiple Apple IDs
Don't turn on iCloud Messages for either device. 

You can continue to use iCloud Backup and include your Messages, if you want them backed-up. If not, leave that off, too. 

As previously mentioned, sharing Apple ID's nowadays will start causing more problems as Apple and developers start using individual iCloud accounts for more individual use. 

Perhaps looking into other photo sharing options to get what you need, until Apple does something, is the better short-term approach.