1 part of Gin
1 part of white rum (or Light Rum)
1 dash of Angostura bitters
2-4 of ice cubes (or ice)
bitter lemon soda
Take a tall glass, put a few ice cubes on top of each other. Pour the Gin and the rum in the glass. Fill the glass with bitter lemon. Leave some of the ice over the surface, so that you can pour the angostura on top of it. It should leave a rather nice track of red down the ice cubes.