The antonym for "acted jointly" is "acted individually." It means that individuals acted separately and independently without any agreement or collaboration with others. When people act individually, they may have different opinions, goals, and approaches to solving problems, which can lead to conflicts among them. However, in some cases, acting individually can have benefits, such as allowing more creativity and innovation to flourish, increasing personal responsibility and accountability, and fostering healthy competition that can lead to better outcomes. Conversely, acting jointly can bring about a sense of teamwork, shared responsibility, and cooperative efforts that can lead to achieving common goals more efficiently and effectively.