The first song is joyful praise for personal salvation. But that kind of salvation cannot be extolled enough being sung by only those who are saved. They need more voices, more praise, more glory. And so the second song demands that we go out and get more voices -
Isaiah 12:4–5 (NKJV)
...“Praise the Lord, call upon His name; Declare His deeds among the peoples, Make mention that His name is exalted. 5 Sing to the Lord, For He has done excellent things; This is known in all the earth.
Let the peoples praise Thee! Let all the peoples praise Thee!