Two approaches come to mind. In both cases, the idea is to stop insisting that the ingredients stay exactly the same in both positions (bottom and top), but instead to make them appear the same. Consider a cocktail that looks like (from bottom to top) A-B-C-B-A. Two ways to accomplish that are:
Use different ingredients on the bottom and top. So your cocktail construction is really A-B-C-D-E. B and D should be the same color, but B should of course have a higher density than C, D, or E. Similarly, A and E should also be the same color but different densities.
Start the same ingredients for corresponding positions, but alter them to change their densities. So, your A-B-C-B-A cocktail is really A1-B1-C-B2-A2, where A1 and A2 are the same spirit, but you've added a lot of sugar or something like that to the base A.