You can't sync text messages from one phone to another without restoring the other phone to the backup of the first. However, a 3G cannot be restored from the backup of a 4S because you can't restore a phone to a backup made on a device running a more recent version of the iOS than the one you are trying to restore to. (Since the 3G can't be updated beyond iOS 4.2.1 you won't be able to restore it to the backup of your 4S.)
If you mean that you want to be able to follow the text conversation on both your phones, you won't be able to do that either. This is a feature of iMessage, which is only available on devices running iOS 5 or higher. Again, the 3G isn't capable of doing this.