You can’t stop syncing and keep the photos on iCloud. You can turn iCloud off on one so they won’t sync together, but the photos will only upload and show to the one that stays connected to iCloud. The photos that will delete are the ones that were synced from the other device, photos on that device will stay. You should download photos to the device before turning off iCloud though, so you don’t lose anything that are in iCloud, but not fully downloaded.
ICloud is not meant to be a Photos Storage, it can be used one way too upload, but not download. ICloud is meant to be a syncing service so that photos and such can be seen from any device on your Apple ID.