Things to Keep In Mind

Heat Rises
As you begin to get multiple tanks remember that heat from one tank will rise to the tank above. 
For example we have a Leopard gecko tank (a dessert reptile) set over a Crested gecko tank (a tropical reptile).  While the excess heat from the Crested light is appreciated by the Leopards if it was the other way around the Crested would not be able to tolerate the excess heat from the Leopards.
Keep convection in mind when you begin stacking tanks.

All Lights Are Not Created Equal
Keep in mind that all reptiles do not have the same type of light requirements.
If you are planing to place tanks next to each other and use one long bulb to light both make sure both reptiles require the same light.  Tropical reptiles generally require less UVB than desert reptiles.
Tropical Lights are generally 5% UVB
Desert Light are generally 10% UVB