Use a single-output floating supply as a dual relative to ground
I'd like to use it to produce something like +12V and -2V with respect
  to the ground of my circuit.


Depending on how much current you might be expecting to take from the 12V (or the -2V) to 0V (ground) a beafier version of the following may be needed.

From the 15VDC uses a voltage regulator to produce 2V relative to the most negative lead on the 15V DC supply. Connect this output to 0V on the rest of the system. 

You now have a -2V rail (formerly the most negative lead on the isolated 15V supply). By inference you also have a +13V rail that can be regulated down to +12V with a low drop-out regulator.

Or maybe you use one of these: -



And then a negative voltage regulator to get -2V