It so happens that the only HID lamps I have are a 400W MH and a 400W HPS.
I have them on separate timers.
I am presently two weeks into 12/12.
The MH is on for 12 hours with the HPS running additionally for 4 hours.
By the end I plan to reverse the pattern.
I am attempting to get the best result for the least power used.
Anybody care to offer ideas on how I might accomplish this with the given lighting?
I think that having both lights going at the same time could be beneficial but I have no empyrical evidence on my opinion. If you want to run both then it partly depends on how big your space is as to how you would run it.
I have 2 cabinets that are 4'x 30"x7' and I run 2 250w HIDs in each one. They are mounted so that they sit at opposing angles from each other. You could run 1 with MH and the other with HPS or you could put dual arc bulbs in both fixtures. If you can get a dual arc bulb that will produce 20% more lumens than what your space calls for then you can use just one light.
Dunge: If you are running the HPS an additional 4hrs then you plants will not flower as they are getting 16hrs of light. I would think the plants would be better served to have the HPS set for 12hrs and have the MH come on for only 6-8 of those 12 hours to give you extra lumens while using less energy than having both on for the full 12. You could simulate natural light of the day by having the MH come on 2-3hrs after the HPS come on then go off 2-3hrs before the HPS goes off. That would closely simulate the high intensity of the middle of the day, give you extra lumens and spectrum, and save some of the energy of being on constantly.
I use both. I have two 150 watt HPS one on each side and a 400 watt dimable ballast that use both type bulbs in the middle.
I use just the MH for veg then turn the MH to 50% and turn on both HPS for the first 2 weeks of flower, gradually turning the MH to 100%. Then I switch bulbs and use all HPS up until the last 2 weeks of flower, then I put the MH back in until harvest time.