1. "Immortals Joining the Three Lights" is a couplet about Taoism, which emphasizes the three talents of "heaven, earth and man" and holds that the three talents are the foundation of all things in the universe. The "three lights" in couplets refer to the sun, the moon and the stars, which means that Taoist immortals shine on the world and bless people's peace and good luck.
2. "A world under one roof" is a couplet about family, which means that a family of it runs in the family, and Xian Yi, old and young, enjoys family happiness and emphasizes family harmony and warmth.
Therefore, the themes and meanings of these two couplets are different, which emphasize the different themes of Taoist immortals and family harmony respectively.