How can I copy/sync SMS messages between iPhones
No. The sandbox that contains app data prevents this as does the restore wiping instead of merging data functionality you are already aware of.


You can back one phone up and restore it to the other.
You can use iMessage to sync things in a forward direction ( new messages can sync well )
You can use a tool like Phoneview to export all the messages from multiple devices and collate / combine / preserve them on your Mac. 


Some commercial transfer products and apps claim to do this, but I haven't seen anything consumer focused on the App Store to do this transfer or merge without jail breaking the device you want to do the "merge" - perhaps someone is more clever or informed and has a "yes" solution.