A Celestial shot cast with Twinbow Terrorcoil’s effect active applies spell damage both on the 2nd Celestial shot cast by Terrorcoil’s effect and on the successive spell cast after it (for a total of +6 spell damage, +2 and +4). Probably the buff granted by the first Celetial Shot should affect only the successive Celestial Shot (it’s “consumed” by it) and not also the next spell cast.