Odd. I looked up the tek fixture. The information I found said they use two Triad ballasts so I looked up the ballasts. The ballasts only come configured for one or two bulbs.
I would suspect the 6 lamp fixture has three, not two, ballasts (possibly four, two two bulbs, two single bulb ballasts). There are only two switches so they could be wired as 4/2, or 3/3. I'd think 3/3, but that would require the four ballasts.
So I have no idea.