I am designing a dual tank system on my truck so I've done some research on this. Gravity flow has drawbacks like Rayra says. If you lose a fuel line, the tank will drain out, that's dangerous. You can't always count on gravity feeding the main tank unless you have it much lower than the feed tank or tanks. You also have to think about the venting system. Each tank will need to tie into the same venting system or have it's own dedicated system. You don't want to have a situation where one tank siphons from the other through the vent line. You also need enough back pressure to shut the fuel nozzle off when filling. You don't want fuel shooting out of the vent hose. The factory FORD dual tank systems have dedicated pumps in both tanks. I have decided to use one tank for storage and transfer that fuel to the main tank with a pump. in a worse case situation, I can swap the supply line to the engine from one tank to the other.