There is no way to get 18V from a single 9V adapter… not even the Godlyke one or the 1 SPOT. Think of it this way… adapters are really like batteries in that they output DC voltage via a positive wire and a negative wire. With batteries, the voltage increases when you run them in series (i.e. When you line up 2 C size batteries in a flashlight, each battery is 1.5V, but when combined it’s 3V). This is because you are running them end to end, negative to positive. You can’t do that with an adapter. You would need two adapters, taking the negative wire from one and the positive wire from the other, tie them together, then take the other wires (positive and negative) and connect them to whatever you want to power.
The point of this is that you need *separate* 9V power sources to combine together to make 18V. Two separate plugs on a daisy-chain cable is not two separate power sources because they’re both coming off the same wires. Now, you could get two 1 SPOT’s and create a Y cable to join them together and make 18V, but that’s a lot of trouble and expense to go through when you can just get an 18V wall-wart adapter from Dunlop (or others) to do the same job. Lastly, IMHO, overdrive pedals sound brittle (overly bright) when powered with 18V, *and* you’re pushing the maximum voltage rating of the op-amps so the lifespan of your pedal may be very reduced.