Post the manufacture name and model number of the supplys.
If no markings some pics might help.
The resistor divider approach works OK if the current in the supply rails
very low, otherwise its best to use a buffer to create the virtual ground,
that avoids supply droop with load.
The above example, examine output current spec, and take care to note worst case power dissipation.
If your load exceeds OpAmp capability then asomething like this -
Regards, Dana.