What arguments support the idea the two birth narratives can be harmonised?

The accounts don't directly contradict each other, so there is no reason why they both cannot be true. Some believe that Luke received his account of Jesus' birth directly from Mary herself. This would explain why he focuses more on her perspective. There are also lots of similarities in the accounts. Both assert that Jesus was born in Bethlehem at the time of Herod, to Mary. They both detail that Mary was betrothed to Joseph, that Jesus was born of immaculate conception by the Holy Spirit