Ok the most obvious and glaring problem is that the smaller tank is lower that the larger tank. All of your water above that tank will spill out of the smaller, lower tank. Period. If you only wanted them to be connected so fish can swim through, that's fine. You'd have to make the tops of both tanks exactly the same heigth. The drilling won't be cheap. To make a large enough hole, 3" expect to pay almost $75-100 a hole. Those bits are not cheap. My local lfs and glass shops charge about $50-75 a 2" hole. A 3" hole would only fit about a 2" pvc bulkhead so realize that in the end you'd have diameter inside of about 1 3/4" for the fish to pass through.
A better idea would be to drill a hole about 2" from the top of the taller tank and have it overflow into the smaller tank. Drill a hole in the smaller tank in the bottom and install an overflow tube 3/4" from the top of the tank. Add a large wet/dry sump under the stand for the water to cycle through. Use a hobby pump to return the water to the larger tank. This would make a loop that would oxygenate the water, cool it a few degrees, and allow ammonia gases to dissipitate into the atmosphere as it is broken up in the wet/dry. You'd be adding the volume of water to each tank and about 20 more gallons to the total volume. Water changes would be much simpler. Only real drawback is that if one fish gets sick it can be transferred to both tanks. Your fish would obviously not be able to "visit" each other either.