Complex sentence: The two brothers and their friend Franz Matsch began working together; by 1880 they had received numerous commissions as a team they called the "Company of Artists", and helped their teacher in painting murals in the Kunsthistorisches Museum in Vienna.
Split into simpler sentences: The two brothers and their friend, Franz Matsch, began working together and by 1880 they had received numerous commissions as a team that they called, the "Company of Artists". They also helped their teacher in painting murals in the Kunsthistorisches Museum in Vienna.