You could always just manually prime the overflows by sucking on an airline. Lets say you are using a U tube for the transfer of water at the overflow. Stick an airline into the U tube all the way up to the bend. Fill both boxes with water so the ends of the tube are submerged and suck on the line. This will start the overflow.
If you want to have an overflow where you don't need to prime. Unfortunately your only option is to have an overflow built into the tank. This requires drilling as shown with zen's previous post. The reason is because the water will automatically flow into the overflow and travel down to the sump. With HOB overflow, once the siphon breaks you're done with the flow. HOB utilize the U tube because water has to travel over the edge of the tank. Where as built in just flows below the water level. No upward travel.
If you want to have an overflow where you don't need to prime. Unfortunately your only option is to have an overflow built into the tank. This requires drilling as shown with zen's previous post. The reason is because the water will automatically flow into the overflow and travel down to the sump. With HOB overflow, once the siphon breaks you're done with the flow. HOB utilize the U tube because water has to travel over the edge of the tank. Where as built in just flows below the water level. No upward travel.
