@Caustic, @sebastian, yeah, it does sound like a short. If both power supplies seem OK until you plug something in, I see two main possibilities.
- There is a short in the flat cable assembly. Especially probable if made at home. If the connectors are crimped on at even a slight angle they can short adjacent pins.
- The inputs of the power supplies from the wall wart are not exactly in parallel. Even though the inputs are AC, the same phase must be applied to the inputs as one side is connected directly to the ground output.
A less plausible possibility. - The flat cable is plugged in backwards at either end, could be caused by
i) Using non-shrouded headers
ii) Mounting the shrouded header backward
iii) Mounting the connector on the cable backward