SPRINGFIELD — Two Chicago Public Schools employees are challenging the validity of their signed contracts to pay union dues as unconstitutional under a 2018 decision that ruled mandatory union dues are a violation of free speech.Lawyers with the National Right to Work organization, which represents CPS employees Joanne Troesch and Ifeoma Nkemdi, have filed a petition with the U.S. Supreme Court that seeks to void the membership contracts signed prior to the Supreme Court’s decision in Janus v. American Federation of State, …