Put the Simple Syrup, lime juice, light rum, mezcal, aged rum and Scotch in a cocktail shaker, add large ice and shake vigorously. Double-strain over fresh ice in a highball glass, add a lime wedge to the rim at 2 o'clock and serve.
Combine the sugar and 1 cup water in a small saucepan and bring to a boil. Cook, stirring, until the sugar dissolves. Remove from the heat and let cool completely. Keep covered in the refrigerator up to 1 month.
This drink is a cheeky riff on the classic rum and Coke. You are using a blend of light and dark rums with lime and sugar (which is essentially a fresh daiquiri) and then adding just a little bit of 2 very smoky spirits: Islay Scotch and mezcal.