Complex sentence: According to legend Castor and Pollux appeared on the battlefield as two able horsemen in aid of the Republic; and after the battle had been won they again appeared on the Forum in Rome watering their horses at the Spring of Juturna thereby announcing the victory.
Split into simpler sentences: According to legend Castor and Pollux appeared on the battlefield as two able horsemen in aid of the Romans. And, after the battle had been won they again appeared on the Forum in Rome watering their horses at the Spring of Juturna thereby announcing the victory.