The disciples ran their errand, returned to Jesus, and spread their outer garments on both animals. Both the donkey and the colt entered Jerusalem. The "them" on which Jesus sat were the garments, not both animals. [source][source][source]
This deliberate preparation for a citywide reception contrasts with Jesus" former approach to ministry. Before He had not drawn attention to Himself deliberately, but now He prepared to do so. He had formerly withdrawn from the antagonistic hierarchy, but now He organized a parade that they could not miss. [1][source]