Gathering together, fasting and prayer; from this stance the Holy Spirit was able to move them in the directions He saw fit. He could have told Saul and Barnabas individually "go" to the same effect. He is dealing with the Church as a body however, beginning to promote it's many gifts and unity. The work and testimony of the Church is much like the work and testimony of OT Israel, they stand together and they fall together. He is thinking in terms of Kingdom and not rugged pioneering individuals. We often fail to listen to the Spirit because we are expecting Him to move us outside/independently of the body. Not many of us would be willing to listen to Him if he decided to speak through another so called prophet or a conference of prophets.
2012 - pBiblx2 Field Wise Bible System Version 2.0.9d - GPL3