In Texas, when there is an old Child Support/custody case from before the Divorce was filed, the old child support/custody case needs to be transferred and joined up with the divorce case. See Texas Family Code 6.406 & 6.407
Child Support cases are sometimes filed by the Attorney General or when parties initially separate. Parties might let these child support or custody cases run their course before turning their attention to the divorce and dividing marital property/debts. It's better to handle all the child-related issues and Divorce all at the same time, but sometimes people just put off the divorce and let the child support and visitation case take care of the immediate need for child support to start. If you settle the child support and custody before tackling the divorce issues, then you might lock yourself in to something you might not want when later dividing assets and debts in the divorce.
If you have an old custody or child support case, and then later file the divorce, you MUST mention the old custody or child support case in the divorce filing. A Motion to Consolidate is the best way to join the two cases up if they are both filed in the same County. If the old custody case is filed in a different county than where the divorce is filed, then a Motion to Transfer, filed in the old county where the custody/child support case is filed, would be the best step. Failure to bring together the older custody/child support case into the divorce might result in delays and increased costs.
For these reasons, it is highly advisable that when you meet with an attorney or attempt to file a divorce on your own, that you disclose prior court actions that might effect your divorce case. It is always easier to tackle such issues in the beginning than to have to back up and redo major paperwork and court hearing dates. If you have this type of problem, just bring your old paperwork along with you to the consultation or at least mention the old case and where it was filed to whomever you are consulting with on your divorce.