Stackable overhead filters work exactly the same way as the normal overhead filter. The difference is scalability. The SOHF media containers are made in such a way that they could be stacked and added onto one another infinately (limited by the power of your water pump as the water have to be pumped higher with each higher stack). As a result, extra biological media could be added to the filter as an when it is needed.
The second advantage of stackable overhead filters is that if several stacks are used, the biological media could be cleaned on a rotation (rotate by stack box and not all at once) without damage to the overall bio filteration.
One thing to be careful is that too many stacks can cause the weight to increase as well. Make sure you have good support on at least 3 points and not all on the center bracing of the tank otherwise the bracing may give way.