Original: This same Synod then added these Canons to two other documents, both of which were in common use by the Dutch Church at the time: the Heidelberg Catechism (1563) and the Belgic Confession (1561).
Simplified: This meeting then added the Canons of Dort to two other documents, the Belgic Confession and the Heidelberg Catechism, which were both commonly used by the Dutch Church at this time.