While I dont know for planted tanks as I've only used halides over reef tanks, but over our 150 (72x18x26) we have two 400W halides, and the light is really even across the tank except where the braces are. The bulbs are 12" off the water. I think you'd be fine with two.
I have a 180 Gallon 6 Foot tank and I was looking at going with only 2 M/H for it. After talking to the guys on here they all recommended 3 m/h as a M/h light gives you roughly a 2sq foot coverage. After I got my 3x150watt AquaOne light I must be honest that I am glad I got a 3 M/H setup compared to 2 M/H setup as even with 3 M/H I still have a slightly less intense light at the ends of the tanks.
The other thing you do is get 2 m/h and just use low light plants at the ends of your tank. Just remember M/H lights are very direct in there light and this is why they are also good at penertrating the depths of a deeper tank.