What is the name of your state (only U.S. law)? Georgia
IS FILING JOINT INCOME TAX RETURN A MANDATORY REQUIREMENT?
Other requirements are met before January 1997.
According to Black's Law dictionary, there is no specific requirement on tax returns, although it states that the parties must live together as man and wife and take advantage of the rights of such. Filing a joint tax return is so important that it seems to me that it would open the issue to challenge if either party wanted to do so. The other party would be forced to produce sufficient evidence to overcome the lack of a joint return - which would not be an easy thing to do.
A Georgia Divorce site specifically lists joint tax returns as one form of evidence that they were living together as man and wife:
Common Law Marriage in Georgia
Overall, it's not 100% clear, but without joint tax returns, you would need pretty strong evidence that you were living together as man and wife. Simply "we intended to spend the rest of our lives together, raised children together, etc., etc., etc." would not be enough. Do you have specific documentation that the couple were living as man and wife?