In your particular case, the answer is: No, you can't.
One can parallel supplies, then the maximum voltage is set by the supply having the lowest voltage, and the current ratings can be added. One has to take some precautions, e.g. diode decoupling the supplies.
One can serial connect supplies, then the maximum current is set by the supply having the lowest current rating and the voltage can be added. One has to take some precautions, e.g. reverse protection using diodes.
One cannot simply average the output ratings. Of course, in your particular case, on can build a switching mode step-up converter to make 25V / 7A from the 15V/60A supply. The other one isn't needed then anymore.