Messages in the Cloud with multiple Apple IDs Background

I use the same main Apple ID for iCloud across all the iOS devices (a couple iPhones and a couple iPads) in my household. However, for one of the iPhones, I use a different Apple ID for Messages/iMessage. This is on my wife's iPhone; she uses her own Apple ID for Messages so that she has her own set of messages and conversations on her phone.

So, on her iPhone, the iCloud info shown at the very top of the main Settings.app view shows my Apple ID credentials. However, down in the Messages settings under the Send &amp; Receive view, her own Apple ID is listed.

Question

If I turn on the new Messages in the Cloud feature — the one that's new in iOS 11.4 — how will this work with the setup I've described above?

My fear is that after I've turned on Messages in the Cloud on my iPhone — and all my messages sync up to my iCloud account — when I turn on the Messages in the Cloud feature on her iPhone, all my messages will sync down to her phone. Or, probably just as bad, possibly all her conversations and my conversations would be merged together in some Frankenstein sorta way.

I'd rather not have to try this in anger in order to find out how it'll work. Either of the scenarios above would be hard to recover. So, I'm hoping someone already knows what'll happen.

Thanks!
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.