ive been looking around at ballasts and stuff lately, i own all this equipment but in the 175w size. i could kick myself for not going 250.
check these out they come with the quick connect cords and are pre wired unlike most ballasts.
you could get the dual 250 and 1 single 250
http://www.championlighting.com/home.php?cat=539
these are priced pretty good, you might find them a dollar cheaper elsewhere.
http://www.aquacave.com/xm-250w-metal-halide-lamp-brsingle-socket-43.html
these can be found 5 buck cheaper elsewhere but you wont find the bulbs cheaper anywhere so to save shipping cost ordering somewhere else it makes sense to buy these here along with the bulbs.
http://www.aquacave.com/metal-halide-parabolic-brreflector-set-w-mogul-base-380.html
everything listed here is good equipment and its everything you need to light a tank with metal halide.
it does seem expensive but the only other way to lessen the cost would be by using magnetic ballasts, which in my opinion is a big mistake on account of the high electricity usage , the noise they make and the heat they produce, and you cant use just any bulb with them, you are limited.
electronic ballasts use at least 25 percent less electricity, are silent the ballasts can be ran at any distance from the tank, they dont get hot,but warm, you can use double ended or single. they are just the right choice.
you asked why two bulbs arent sufficient to light a 6 foot tank.
its because a halide bulb at its suggested distance from the tank wont completely light a lot of area, you would have dark spots and irregularly llighted areas in the tank, you would see shadows or lines of light in it.
you need a bulb every 2 feet.
you have to bite the bullet with lighting, beleive me i looked for months trying to go cheapola on lighting, it doesnt work if you do it the correct way.
i would buy 20k xm bulbs , they rock and you dont need any other bulb for a nice blue look.