The apostle Simon is also called the Cananean (Mark 3:18), or Zelot (Luke 6:15) in the New Testament. Perhaps, he used to belong to the Jewish religious sect of the Zelots before joining Jesus. He is not to be identified with Simon, the brother of the Lord (Matt 13:55; Mark 6:3). Nothing certain is known about his further life.
Jude Thaddeus (Matt 10:3; Mark 3:18) is called son (or brother) of James) (Luke 6:16; Acts 1:13). He is to be distinguished from Judas, the brother of the Lord (Mark 6:3). We also have no certitude about his further life and work.
Shared veneration of both apostles is not attested in the Eastern Church. Their common feast day reached from Gaul to Rome relatively late.