Hi, I just asked my boyf who used to be a bartender... Not sure if you mean cocktails or shots? More shots combinations require layers. Different alcohols have varying densities. So for example is you wanted to make a B52 shooter made up of Kahlua, Baileys and Grand Marnier. Pour Kahlua which is of greater density into the shot glass. Then pour the Baileys into the shot glass but to do this, hold a spoon into the glass above the Kahlua and pour the Baileys onto the back of the spoon so it drizzles into the glass ontop of the Kahlua. Then with the Grand Marnier, because it has similar but lower density to the Baileys repeat with the back of the spoon but very carefully. I've attached a list of the densities of alcohol for your cocktail making pleasure.
http://www.*****************.info/Bartending_I nfo/Density_Chart.html
If you do mean cocktails, e.g Tequila Sunrise with orange juice and Tequila. You just pour the Grenadine Syrup slowly down the side of the glass so that it falls and rests at the bottom of the glass.