Sorry for causing confusion, I tried to be detailed ended up making things a bit hard to explain, but you kind of got it anyways, to put this in even a simpler form, just plug the second PSU into an outlet, take 3*6pin cables and plug them in the 3rd board which now is unplugged.

You could also connect the control board to the second PSU instead of connecting it to the first one since the second PSU will have a lot more room, the control board doesn't consume much but still worth it IMO.