how to separate two icloud accounts on a shared account

It won't go back the way is was. Something in IOS 12 changed what happens when 2 people share an Apple ID. I suggest one of you create a new ID to separate your data. There may be some data loss, but you can minimize it. If syncing photos with iCloud, ensure Optimization is turned off in Settings/Your Name/iCloud/Photos and/or in Photos/Preferences/iCloud. Then make sure full resolution photos are on the device/computer. When signing out of iCloud, if asked whether you want to retain data, choose yes. Messages conversations will be lost if they are addressed to the ID. See the article below for other places that the old ID should be removed. Also set up Family Sharing to keep access to your apps/purchases or ask your parents to set it up. Also see Winston Churchill's response in the below discussion

Apple ID – Create

How to transfer all files from one iCloud account to another

 Apple ID email address or password - What to do after you change

Family Sharing

Family Sharing - Set up

Family Sharing - Use your own Apple ID for Family Sharing

Family Sharing -What types of content can I share?

Family Sharing - If you don‘t see your family‘s shared content

Family Sharing - purchases and payments

Family Sharing - Hide and unhide music, movies, TV shows, audiobooks, and books

Family Sharing – share iCloud Storage

Family Sharing - Share an iCloud storage plan with your family

Family Sharing and location

so how do we separate our iClouds without erasing pictures, etc, and have it go back to the way it was?

This is confusing.

Back to the "way it was" means sharing your Apple IDs and all content.

You can't do both. You can't share and be separate at the same time.

I have 6s and xMax with the same Apple Id and normal the same iCloud but problem was that on xMax i could not see my documents was stored by 6s and computer before / just his new iCloud documents .

I ve buyed an iPad 10 and use with the same Id Apple but he only see iCloud of 6s and computer - Not xMax !

My Solution was :

sign out 6s and make a new Id for it ,

sign out xMax and iPad 10 and sign in again after some min with same ( initial ) Id .

Next day i could use the same iCloud on both devices

My conclusion now after i checked differit assitents , is :

Two same devices can Not use the same iCloud space .