Metal Halide bulbs put out a bluer light spectrum that is most needed for plants during their vegetative growth stage. Using one will give you tighter nodal spacing (less stretch than will be seen using an HPS bulb alone) and thick lush growth.
　
High Pressure Sodium bulbs put out more light in the red spectrum range, that that is most needed by plants while in the flowering growth stage, but if used for vegetative growth will cause wider nodal spacing and growth that is not as thick and as lush as a MH bulb would give.
　
What can be best is a combination of both light spectrums. Some wattage HID lights can use a dual-arc bulb that puts off both a MH and a HPS light spectrum from one bulb at the same time. An example is a 100-watt dual-arc bulb is a 500-watt MH and a 500-watt HPS in one bulb.
　
They are not available in every wattage though. I have seen them in 1000-watt, 600-watt and 400-watt but lately for some reason I cannot find the 400-watt bulb offered so I do not know if there was some design problem and it is no longer offered or if the sites I had found it on in the past just no longer offer it possibly due to lack of sales.
　
If you can use one, it would be a wise option to take. If someone is burning lower wattage HID lights and can deal with a slight increase in temperature adding a second light of the different light spectrum is a good option to take.
　
If someone has a simple batwing design reflective hood they can remount the single light fixture to one side and then add a second light fixture, which would come with another ballast, to the other side of the batwing reflector. For wider coverage again if they use simple batwing reflectors they can attach the two reflectors by drilling small homes near the corners of where the two reflectors would touch if put together and slightly overlap them and use a sheet metal screw or screw and nut to hold them together. The drawback of that is some plants will mainly receive one light spectrum and the others will mainly receive the other light spectrum. The two light spectrums will not mix as well as when combined under one reflective hood. That means you need to rotate your plant positions like every other day or so to insure they receive an equal amount of light of both spectrums of light throughout growth.
　
If someone uses a different style of reflective hood depending on its size and design a second bulb of a different light spectrum can be added. Some reflective hoods are designed to accept a second bulb though most are not.
　
If someone uses CFL’s it is just a matter of purchasing ones with different Kelvin temperature/color ratings and then spreading them out so their plants receive as equal of an amount of each light spectrum as possible.
