Identify the highest voice. Whether female or a male tenor, this voice has characteristics that make it different from the other singer. Focus on this voice as you listen to the track through your equalizer. Adjust the frequency you want the treble knob to single out. You will hear the voice "pop" out of the mix when you find the right frequency.
Focus on the lower voice. Play with the frequency knobs until this voice pops. You will notice that adjusting the lower voice frequencies affects the higher voice as well. You can't help this. You can, however, choose a good compromise, where the lower voice stands out and you don't harm the sound of the upper voice too much.
Adjust the mid range. This is probably the place where the voices blend as they share this frequency. You will find a tone as you turn the mid range knob through the frequencies that emphasizes the blend of the voices.
Raise and lower the volume for each of the three frequencies you chose. Each of the frequencies you choose has a volume knob associated with it. This takes more art than science. You may find places in the song where you want the high voice emphasized, while other places feature the low voice. Still other places, especially the chorus, may call for more volume on the blended tones you found in the mid range.