Q: My 2 daughters have itouch, both on my same apple account. How do i enable them to text each other .

Each daughter needs their own Apple ID to send and receive messages independently.  Apple intends that Apple IDs (accounts) be for an individual and not shared like you daughter are doing.  Two people sharing the same ID causes all kinds of problems as you are finding out.

However, Family Sharing allows people within the same family (up to 6 members or Apple IDs) to share purchases like apps, videos and music. Separate Apple IDs will allow each girl to  tailor their content to the individual, which will become important with the child sooner than later.  You can then access most content that each other has purchased.    Set up Family Sharing - Apple Support

With the current shared Apple ID as a separate family member, each daughter and you can download their favorite content from that shared ID.  The idea is that each family member has their own tastes and may not want all that the master ID contains.  Yet with family sharing, if family members want to share, each can access each other's content.  Following this concept, when the child is ready to go into the world on their own and start their own family, they can take their music with them.  So I would let the current shared Apple ID whither over time.  Over time it would potentially become a behemoth everyone has a hard time searching in for their stuff.  Also there is no privacy in the shared Apple ID.  These may be important for a growing family that doesn't necessarily want common knowledge of each others interests. 

To share purchases between family members:

iTunes Store

When logged into your own App and iTunes store Apple ID, In iTunes Store app on iDevice, from the iOS device you want to add content too, choose More at bottom right of screen > Purchased > Choose Family member that purchased item > Choose type of purchase > Choose item > click cloud icon with down arrow to add to shopping family member's device.

App Store

When logged into your own App and iTunes store Apple ID, In App Store app on iDevice, from the iOS device you want to add an app a family member purchased (including master Apple ID), Choose Updates from the lower right corner > Purchased from top > choose Family Member who purchased app, Find app in list and click Free or cloud icon with down arrow.

I'm sorry.  I just noticed your tag lists an iPod touch running iOS 5.1.1.  The information above is for iOS 9 and above.  I should have asked what your daughter's iPod Versions are and what iOS they are running, though the information about needing separate IDs to message each other is still valid, older iOS would require somewhat different steps.

They can still send messages to each other under iOS 6.1.6  if they use their own Apple IDs for Messages.  New IDs  can be obtained at https://appleid.apple.com/.  Once the IDs are set up, you will go to General > Messages on each device and enter the Apple ID by clicking Use Apple ID as memory serves.  Or if both devices are using the same Apple ID currently, you will have to sign out first.

I would add the new IDs to iMessage and Facetime and leave the shared Apple ID for app purchases in iTunes and Apps Store.  The girls could use their own, new Apple IDs for iCloud for privacy for pictures and separate contacts, calendar, email sycning or they could use the master Apple ID if they want their data on their devices to be the exact same.  Personally, I would change this to the new individual IDs. 

This article shows the screens you should be seeing.  If you need help let me know, though I don't have and iOS 6 device anymore and will be playing from memory.