Here's what Cioks have to say about the Series Adapter Flex:
By using two isolated outlets the Series adapter Flex gives you the sum of voltages of the two outlets used. This adapter Flex cable should be used in combination with any of CIOKS standard Flex cables if you want to power an 18V, 24V or even a 40V pedal. It gives you not only the higher voltage but also the possibility to choose the type of plug at the pedal end plus the length you need. The plug type and length is determined by the standard Flex cable used with the adapter Flex. If you’re looking for a simpler way to power an 18V pedal with a standard 5,5/2,1mm centre negative DC plug (BOSS standard) please have a look at CIOKS Stack Flex.
Please note that the Series adapter Flex makes a series connection of the two outlets used. These two outlets must be isolated from each other. You get the sum of voltages (V) and not the current (mA). If you use it with two 9V outlets with a 100mA rating each you’ll get 18V and still a max. current of 100mA. If you use it with two 9V outlets with 200mA each you’ll get 18V and max. 200mA. If you use it with two outlets rated to max. 200mA set to 12V you’ll get 24V and still max. 200mA. BE CAREFUL when using Series adapter 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, you’re welcome to send an e-mail to firstname.lastname@example.org and get some useful information.