Complex sentence: The tradition of riding with double reins in one hand is preserved by polo players, where double reins remain the norm but the double bridle has been largely replaced by the pelham bit or the gag bit.
Split into simpler sentences: The tradition of riding with double reins in one hand is preserved by polo players. In polo, double reins remain the norm but there too the double bridle has been largely replaced by a pelham bit or a gag bit.