I don't think there is a short, snappy way of expressing this in Spanish. "Limited Purpose Married Joint Tax Filer Agreement" appears to refer to a "Contrato con efecto limitado entre cónyuges casados que presentan la declaración tributaria conjunta."

Also you might need other wording if this is for an Argentine audience. In Spain a "joint tax return" is called "declaración conjunta," but this may be different in Argentina.

"to file a tax return" = "presentar la declaración tributaria (or) presentar la declaración de la renta"

Hope some of this helps.