I would be inclined to buy the 8 bulb.
I agree with this. The 2 bulb is going to spread the light into areas that the plant won't be using. A tighter (shorter 2' or 4') fixture with more bulbs will give you better intensity. Try to keep the light the same size/shape as the area that your canopy will cover.
For 2 plants, I'd consider the 2' 4 bulb if you want to keep them small, or the 4' 8 bulb if you're looking for some big bushes.
I'd also consider some CFLs or better yet t5ho single bulb strip lights for side lighting. It will significantly increase your yield and lower bud density. t5's run cooler and produce more usable light than a shitload of cfls.
The key to flouro's is to keep the lighting close to your canopy. Side lighting gets the lower stuff, the tight top light gets the top.
Definitely consider (as knally stated) mixing red and blue bulbs in these systems, or switching from all blue to all red at 12/12.