Deleting photos from one iPhone also deleted from my other iPhone

You are on the right track. However, it is not necessary to sign out of the Apple ID; just turn off Photos in Settings/[your name]/iCloud on the other phone. This will stop syncing.

If you want to sync photos between devices and iCloud then you will always have exactly the same photos on all devices. Additions, edits and deletions will be mirrored across all devices as well as https://icloud.com. If you want different photos on different devices then don’t use iCloud; instead use a different way to transfer photos to your computer or other devices. Recommended methods are covered in this article→Transfer photos and videos from your iPhone, iPad, or iPod touch to your Mac or PC - Apple Support

You can (and probably should) also back up photos to outside non-iCloud online services, such as Google Photos, Amazon Photos, Shutterfly, Dropbox, Box.com, Livedrive, etc.

If you sync anything (Photos, contacts, etc) to iCloud, then, when you delete something from one device iCloud will sync and delete it from all other devices sharing the iCloud account.

Don't Quote me here. I am not an tech or Apple Professional but I think on the phone that you do not want the pictured deleted from if you sign out of your Apple Id from that phone than the pictures will not be deleted. But watch out with that because once you sign out of your Apple ID on the device you do not want to have the photos deleted from than it will no longer get new photos downloaded to it. Now I could be wrong about all of this and sound like an idiot here but I hope I am helping.