As you have stated that you had quit the company, this implies that you are no more a director of the company and therefore not authorized to sign any documents. Signing of any such documents post your resignation shall be unauthorized and liable to be challenged in the Court of law. For you to be an authorized signatory, they shall have to reinstate you in the Board of Directors and only then will your signature be valid.

With respect to your dues, you need not worry as the Company is under a legal obligation to pay you your dues and they cannot evade the same. You do not need to give in to the threats as you have a legal remedy of filing a recovery suit to claim your dues in the Company. We suggest you send them a legal notice claiming your dues within 15-30 days and if the same are not cleared, you can approach the competent court.

Now as you have mentioned that you being 1 out of the 2 directors in the Company and you have resigned, we would to inform that in case the company does not find a new director for the company, it shall be liable to be wound up as the Company Law requires the minimum no. of directors in a private company to be 2 at all times.