There could be federal charges even after a conviction or acquittal on the state charges, so the answer to part of your question is yes, it is allowed. You fiance needs a good federal defense lawyer to represent him in the new case. Federal charges often have stiffer penalties than state charges for the same conduct, and federal practice is very different from state court. Your fiance should consult with a good federal defense lawyer as soon as possible. I am not licensed in Texas, so this answer is for informational purposes only.
There are many charges which are violations of but State and Federal laws. Unfortunately, one can be prosecuted under both sets of laws - state and federal. Most times, if the feds pick up a case, the State will drop it - but not always. And, as along as they are within the statute of limitations, they can pick up a case at any time (even if the case has already been pled out in state court.)
Make certain that the lawyer you hire has practiced in federal court. It is a different animal in federal versus state court.
If your boyfriend trusts the attorney whom the court appointed to represent him, there is no reason to hire a different attorney. Federal courts carefully review the applications of attorneys who want to be appointed to represent indigent people in criminal cases. The clientele of these attorney will often include both court appointed and retained clients. Generally, the attorney will provide the same service to clients whether they are retained or appointed. If however your boyfriend is not satisfied with the court appointed attorney he should find money to retain a private attorney of his own choosing. Most courts will not allow defendants to select which attorney the Court appoints and will not appoint a different attorney absent a conflict of interests or misconduct. Good luck.
THESE COMMENTS ARE NOT LEGAL ADVICE. They are provided for informational purposes only. Actual legal advice can only be provided after consultation by an attorney licensed in your jurisdiction. Answering this question does not create an attorney-client relationship or otherwise require further consultation