There are two answers to this one kind of:
1) Keep in mind that if you make a new backup in iTunes now, you're likely to overwrite the previous backup. iTunes usually only keeps one backup at a time for each device.
2) The messages will not blend together -- the new backup will have the newer text-message database file from your phone now.
If you're looking to recover deleted text messages, you may want to look into some third-party software for this. I am the lead developer on Decipher TextMessage a program that will read your iTunes backups of your iPhone and
a) do the blending together of different backups to create a wholistic history (assuming you do have multiple backups -- if you're interested in how to keep multple backups, drop me a note here or on our website!)
b) allow you to view all of the messages that were not deleted in your backups
c) help you recover deleted messages from the backups. See that other forum post I liked above for information on how that is possible 🙂
Disclaimer: I work for Decipher Media, so I stand to benefit from the links I posted above to the Decipher Tools website.
<Edited by Host>