You're losing it ski...

Most good PSU's have floating outputs, so until you link them together they are all independant. Lots of PSU's out there with single, dual and tripple variable outputs. All those outputs are completely independant... nothing to say you've got to strap all the Black terminals together.

So, just connect the Red +ve Terminal of one output to your Common Ground (0v) point and the -ve Terminal then becomes your -ve rail (with respect to the +ve terminal). So, just dial-in 28v and you've got your -28v Supply. Ditto for all your other rail requirements, positive or negative... Black or Red, all depends on which one you connect to 0v.