Or one to one where the first is a spike that will take time to determine what should go into the second (discovery, learning, experiment, further estimations). You would not want to commit to a unit of work without some idea of what it entails in Agile. The alternative is Programming Mother Fucker where you just dive in and see where it takes you. The business side usually prefers more predictability. The developers usually prefer to Just Getting It Done (tm).