According to the Pennsylvania Department of Revenue website, the Department does not require proof of marriage when a joint tax return is filed, so a joint return may be filed:
Can I file a joint return if I am married under Common Law?
The concept of "common law" marriages is one that arises fairly often and has been the source of a number of Court decisions. A PA Supreme Court decision essentially said that the concept of common law marriage was no longer applicable because there were no longer any impediments to marriage.
Effective 1/1/05, and later, Pennsylvania no longer recognizes common-law marriages. This does not affect common law marriages entered in to prior to 1/1/05.
PA does not require proof of marriage when a joint tax return is filed. PA income tax is an individual tax and if you choose to file a joint tax return please remember that on a joint return, both parties are independently liable for the entire amount of state tax due. Joint filing is for convenience and there is no tax benefit derived for filing a joint return.