Bench DC Power Supply
Unless the supply has separately adjustable positive and negative voltages and currents, it probably is a unipolar supply with a separate earth ground, and can only supply a single voltage. For example, this one (randomly selected eBay listing): 



If you want a second negative supply, you can buy another power supply and tie the (+) output of one to the (-) output of the other, and typically you'd connect the earth terminals together (usually the output floats with respect to earth ground). 

You might want to tie the earth grounds to the common between the supplies (but not always).