Paul traveled to Ephesus after Apollos had left. Later, Apollos returned to Ephesus while Paul was still there (1 Corinthians 16:12). The two men had different personalities, gifts, and roles, but God worked through both of them.
Some Christians in Ephesus did not fully understand the Christian faith (Acts 18:26). They did not receive the Holy Spirit when they believed the good news about Jesus. They had accepted John's baptism, which called for repentance from sin, but had not received Christian baptism, which included the gift of the Holy Spirit. When Paul had instructed them further, they were able to receive Christian baptism and the Holy Spirit (19:5, 6).