Complex sentence: This race marked the beginning of the so-called "B-B" duo-that is, a Bobet-Barbotin partnership where Barbotin became one of the principal right-hand partners of Bobet, especially with regard to the Tour de France.
Split into simpler sentences: This race marked the beginning of the so-called "B-B" duo-that is, a Bobet-Barbotin partnership. From that point forward Barbotin was one of the principal right-hand partners of Bobet, especially with regard to the Tour de France.