Can two baptized heretics have a valid marriage? Yes, but...
If he wasn’t Catholic, he was never married.
Patrick McCreight
MHFM: No, you are wrong. Non-Catholics (whether baptized or unbaptized) who were not baptized into the Catholic Church are not bound to the Catholic form of marriage. Thus, when they contract marriage among themselves it is valid. That is specified in the 1917 Code of Canon Law (Canon 1099.3). Two baptized heretics (who were never baptized into the Catholic Church) would also have a sacramental marriage (although it would be contracted in sin).