My big question here would be "WHY?" I have plenty of planes that I use a 3 line setup on, but if the truth be known I will strive to use a 2-line setup whenever I can. I subscribe to the K.I.S.S (Keep It Simple - Stupid) theory in building planes. If I can make do with a 2 line setup there is no need to go through the added hassle of a 3-line setup.

But to answer your questions above. Yes, you can have two clunks in the tank. The first one is on the fuel pickup line that goes to the carburetor. The other clunk is on the fuel fill line. This will allow the fuel to be pumped out of the tank. You can forego the clunk on the fill line and put a simple piece of tubing bent down to the bottom of the tank at the front of the tank. To pump out the fuel simply lift the plane up on it's nose so the fuel flows to the front of the tank.

The only time that fuel will come out the carb when fueling with a fuel dot is if you have a T-fitting in the fuel line, with the "T" hooked up to the carb, tank, fuel dot. But you can simply close the carb before pumping in the fuel. In a 3 line setup this usually isn't a problem because overflow will go out the muffler pressure/vent line first.

Ken