Are your lighting requirements based on the needs of your plants or just to bring the natural color out in your koi ?
If it is just to bring out the colors, then it really just boils down to personal preference. 2 single strips should do fine.
If it is for your plants, then it depends of their requirements.
This site http://jpsweb.net/plants.html
Low Light Plants: 1.5 to 2 watts/gallon of water
Moderate Light Plants: 2 to 3 watts/gallon of water
Bright Light Plants: 3 watts/gallon of water
Or in the case of your 120gallon tank;
Low Light Plants: 180 to 240 watts
Moderate Light Plants: 240 to 360 watts
Bright Light Plants: 360 watts
I hope this helps.