That makes sense, I understand now. I definitely would prefer having one single psu for both -16 and +16, unfortunately, I didn't find any psu with multiple outputs where -15/+15 can be adjusted to -16/+16 without external stuff.
If I use two separate PSUs, does the -16 PSU need to be as big as the +16? When I look at the photo David Fry posted (http://n8vem-sbc.pbworks.com/w/file/76170281/psu_detail.jpg), it seems that the -16V comes from a much smaller PSU than the +16.