Mix the flavor syrups: In a small cup, stir together the coconut syrup and vanilla syrup until fully blended. The mixture should look slightly thickened and smell faintly tropical.
Add blue raspberry to glass: Pour the blue raspberry syrup into the bottom of a tall glass. Let it pool flat before adding anything else.
Fill glass with ice: Add ice slowly on top of the syrup, filling the glass about three-quarters full. Pouring gently keeps the blue syrup from splashing up the sides.
Pour in the soda: Slowly pour the chilled lemon-lime soda over the ice, aiming the stream at the ice cubes rather than straight down. This should take about 20 to 30 seconds and keeps the layers mostly distinct.
Pour the coconut vanilla mix: Slowly pour the coconut vanilla syrup mixture over the soda, again aiming at the ice. You'll see it float in a pale layer above the blue raspberry base.
Float the cream on top: Hold a spoon upside down just above the surface of the drink and pour the half-and-half over the back of it. The cream should spread into a soft, cloudy layer on top instead of sinking.
Garnish and serve: Top with a maraschino cherry if using and add a straw. Serve immediately so the soda stays fizzy and the layers hold their shape.