Can I file a joint bankruptcy case with my common-law spouse and just pay one bankruptcy lawyer fee and one filing fee?

Yes.

In Texas a "common law" or informal marriage can be just as valid as a regular formal, ceremonial marrage. It is even "codified" or made into a Texas law as Section 2.401 of the Texas Family Code, Proof of Informal Marriage. 

So it is permissible to file a joint bankruptcy case with your common law spouse and save money over filing two separate bankruptcy cases, so long as you have an agreement to be married, you hold yourself out to others as married, and you are living together or have lived together. As an alternative, you can sign and file a "Declaration of Informal Marriage" as provided by the Family Code.

J Thomas Black
Board Certified, Consumer Bankruptcy Law- Texas Board of Legal Specialization