I think 2x1000W will definitely need cooltubes. You're talking about serious radiant heat in a very confined area. Some people may tell you to simply put a fan under the bulbs to blow the heat away. Seems good in principal but in practice, not so effective with 1000 watters. The heat doesn't simply disappear, it's still in the room. You'll have to be dialed in on ventilation.
A fan blowing up will help, but not near as much as aircooled lights. I've tried it both ways and found a combination of the two works perfectly. While the cooltube carries away a lot of heat off the bulb, the fan helps keep leaf surfaces much cooler. You'll have to watch out for leaf dehydration, so keep the fan speed somewhat low.
Are you planning on stacking the bulbs or hanging them in a stadium arrangement? Stacking 2x1000W will grow some enormous trees if you have the overhead space. You'll probably need about 10 feet vertical clearance. Doubt you'll be able to fit 8 plants though, more like 4 or 5 monsters that are about 3 feet wide. In a stadium setup, you can probably get 8 plants in there. I think this stadium setup would work well with 2x100W. Although I'm curious how stacked 1000 watters will work, I can't see the advantage over 600W in a small confined space unless they're spaced in a wider circle. With the extra distance you'll lose the light intensity advantage of the closer 600W bulbs. Maybe you can prove the theory, it will be interesting to watch.
I grow with 3x600W in an oval configuration and get 10x 5 footers in there for each bank of lights. It's still kind of tight so I think I'm going to veg another week and flower eight next time.
Hope this helps,
Fred