Bring 1 cup water, the sugar, orange flower water, and kumquats to a boil in a small saucepan; boil for about 5 minutes. Remove from heat and let cool, then remove kumquats (discard liquid). For each drink, muddle 1 kumquat in a cocktail shaker. Add gin, dry and sweet vermouths, and both bitters; fill with ice. Shake, then strain into a coupe and garnish with kumquat twist.