Nah bro, you won't get your pound, even if you were to grow them taller. Taller growth means less intensity for bottom-growth. You end up with bigger, fatter buds, but less quantity.
And combining the HPS and MH lamps should increase your overall lumen output. Also, by combining both bulbs, you create a Hortilux-type of bulb that covers both blue-and-red lights of the spectrum.
So, they will get taller and they will be denser (at the tops).
If you want to go for quantity over quality, then add another light vertically so you can cover some of the undergrowth. Mylar does an ok job, but not extremely well.