Positive and negative voltage regulation
This will not work, you will be shorting the output of the top converter module.

I have drawn the short circuit with the orange trace:



The - input and the - output of these modules are connected so you cannot re-reference the outputs like you are doing.

What you need are converters with an isolated output or a module that will directly output -18 V / 0 / + 18 V. Have a look here.