1. Sync the phone you want to copy text messages from. Make a new backup.
2. Go to the folder with your backups (google it if you dont know)
3. sort the folders by date modified to find the one containing your the backup you just made (this step is only necessary if you have multiple backup folders, such as if you've synced more than one phone on that computer)
4. Make a copy of the backup folder in case anything gets ****ed up
5. copy the following files from that backup folder to a new folder
-status.plist
-manifest.mdbx
-manifest.mbdb
-manifest.plist
-info.plist
-3d0d7e5fb2ce288813306e4d4636395e047a3d28
6. Set up the phone you want to transfer texts to as a new phone (or do a fresh install)
7. make a new backup after it is set up
8. go to the folder with the new backup created in step 7, delete everything in it leaving an empty folder
9. Copy the 6 files listed in step 5 to the empty folder from step 8
10. right click the phone in iTunes and restore to backup
11. You should have a fresh phone, set up new but with all of your texts saved
It might give an error message when the restore finishes, but don't panic; take a look at your text messages and they should be there.