Try putting the lights BEHIND the plants. You'll have to mount them horizontally, but this way the plants will get the extra light without being shaded from the sun.

i would choose the artificial light over the sun light. you keep it just above the tomatoes. if you let them grow mainly with the window light they will grow towards the window and thus cause leggy plants. the object of putting the grow lights so close to the plant is to keep it from getting leggy. which is the whole point of using grow lights anyway right?

I have seen a post here about the amount of energy that is rec'd depending on how close the light is. And, as far as a laymen can tell, inches count, as in the prescribed few. Again, I save seen posts in here that the suns power far out-weighs that of lights.

Go fish.

Direct sunlight is many times stronger than your lights, so don't block it. Don't be too worried about blocking indirect natural light, especially indoors, because the fluorescents will be more intense if you can get them very close. Ideally, use an arrangement like jkirk describes. Or look for a good reflector system so you can mount the lights further away.