For the top tank, does it have equipment on the back of the tank? It looks similar to the biocube. If you don't want to drill your tank (best way), then consider getting a CPR overflow. Read as much of the bad reviews as possible before you buy it to learn all the inherent risks.
Water goes through the overflow and into the bottom tank. Then you will have a pump that pushes the water back to the top tank. You should look into getting a return U-tube, such as U-Tube with Directional Return
What are you planning to use the bottom tank for?