You mean you LOSE 80 percent of the weight when drying. Heh. I wish my dry weights were 80 percent of my wet weights!

And yes, if you add a second light, even though it's MH, you should get much more yield. Probably not double, but likely not far from it. As to whether it's worth it or not, that's a question only you can answer. Is more yield worth the extra expense to you? I grow more than I can use with one 400 watt light, personally, but then again, I don't sell, and don't want to be in that 'line of work'. If you sell, then it's almost certainly worth doing anything you can to increase your yield. If it's for personal use, then you'd have to smoke like a pot-starved hippy who's been trapped on an island without weed for 5 years to smoke all that one 400 watt light can produce. Well, okay, maybe not that much, but still!