HPS is fine for vegetative growth and flowering as well. Many growers will often use HPS only for veg and flower. Problem with HPS is it allows the plants to stretch. Benefit of HPS is incresed lumens per watt and lower bulb temps.
Metal Halide is pretty much only good for veg, although you "can" flower with them, it is not recommended. Problem with MH, Higher burning temps, not good for Flowering Alone, and lower lumens per watt vs HPS. Benefit of MH is compact bushy growth, and decreased distance between nodes.
SO my conclusion to this subject is.....
Using HPS along with MH during veg stage gets you incresed lumens over using 2 MH.
Using 2 HPS during veg stage will give you increased distance between nodes.
Using 2 MH would be "normal" in veg.
Using HPS along with MH in the flower stage helps to keep growth compact and bushy but you could be getting more lumens and less heat in there with a 2HPS setup.
I would use the HPS MH combo for veg but switch to 2 HPS for flower.
However I would not consider this an advanced growing technique.
