Our 18V Adaptor doubles the 9V DC input to 18V DC on the output. Some power supply designs offer 18V by connecting two 9V outputs together using a special cable, but for this you need a power supply with isolated outputs. Since isolation is only needed occasionally (and when you do, you can use one of our isolators), this is money wasted for most people. Furthermore, it uses two of your limited number of outputs.
Instead, we've made this specially designed 18V adaptor, that uses just one output, and will reliably power any 18V pedal with a current draw of up to 30mA.