Make 18V from a single 9V outlet. It's equipped with a step-up converter in the mould placed in the middle of the cable. When you connect the Booster Flex to a 9V outlet, you'll obtain 18V on the other end, which is a type 1 plug - a black 5.5/2.1mm centre negative DC-plug.
BE CAREFUL when using Booster Flex. There is a risk of damaging one of your 9V pedals by powering it with too high a voltage. Check the specifications of the pedal and if you still have some questions, please get in touch.