Yeah, grounding is key. When i made up my power supply using the same DPS-600PB units I insulated the relevant PCB mounting stud and cut a couple of wire jumpers so that the second unit in the series pair could be floated at +12V while still having it's case grounded to earth.
The 'hope for the best' solution is just to disconnect the earth on the second unit but that way if you get a fault then you could be electrocuted, which isnt very nice at all
Not an issue if you are just using a single 12v unit of course.
I like the way you have used female bullets to hook up to the hot swap connector, much neater than soldering in wires.