Add both rums, simple syrup, lime juice and grenadine to a cocktail shaker.
Add ice to the shaker until it is ¾ full. Shake hard for 7 to 10 seconds.
Strain into a chilled large coupe glass. Garnish with the dehydrated orange slice.
Notes
Make a simple syrup by bringing equal parts sugar and water to a boil in a saucepan. Cook for 2 minutes, until all the sugar has dissolved, then let cool completely. Store, covered, in the refrigerator for up to 1 month.