It would be useful to know which plants you want to grow, but the majority of annual seeds require high light even when very small. Typically they will tolerate full sunlight even before the true leaves appear.

My short answer is that you need another two tubes.

Your two-tube fitting probably has the tubes about 6" apart, giving fairly good coverage of a 10" tray assuming the two trays are placed end to end under the lights. The tubes can be placed an inch or two from the top of the seedlings. If you add two more flats then you either have an arrangement that is 84" long and so is longer than the lights, or it is 20" wide and you can't put the tubes close enough to the seedlings and still evenly light the whole width.

I'm assuming that you are using the 40W bulbs that typically go in such a fitting. Even if you have stronger bulbs then it would be difficult to get even coverage. The link shows that you get 1,500 foot-candles at 3" below a 2x32W light. You should consider this a minimum light level for annual flower seeds. With four tubes you should be able to get all the seedlings at more or less 3" from a tube. You can also use daylight from a window to increase the light. Light levels at a south-facing window, but not in direct sun, are around 1,000 foot-candles. In the sun, they will be more like 10,000 foot-candles. Artificial lights quickly become irrelevant in these conditions and may just be blokcing more light than they create.

Here is a link that might be useful: Foot-candle measurements

I have my shoplights set up and then went to Wal-Mart and got 2 emergency blankets that they sell for about 1.97 in their camping department. I surrond my trays with this and the light has really increased - as well, it keeps them a little warmer!

Pintail,
You don't say how wide your shoplight fixture is. The fixture and reflector should be about as wide as the area you want to cover. If it is a small shoplight it might only be 6 inches wide so you should have 3 or maybe even 4 fixtures to cover the space. The seedlings will appreciate the extra light and will be stocky and healthy.
If it has no reflector then you can easily fit three in the space you have, but a reflector is more efficient and you should have fixtures with reflectors. If it is a large fixture with a big reflector you will have trouble getting more light on your plants because you won't be able to fit another fixture next to it.
Bob B

sorry I haven't mentioned this earlier. I have seen ads for reflectors, and they come in all materials and textures. And in a post about mirrors or paint, it is just the overall efficiency of the reflectivity, not even smoothness, which will probably be close no matter what you choose.

You could just buy some loose sheet metal, aluminum or galvanized, either in a roll, or flat. You should have many options. You can cut it no-problem with scissors, bend it on the edge of a table, or possibly clamped between two lengths of wood. You could drill and screw it on, or use a glue like JB Weld, which which actually hold a cracked engine block together for a time. If you don't have a drill, a hammer and nail to drive holes, and small screws and washers will do.

Pintail, look for another $4.46 Commercial Shoplight from H-D, #140-904. And use T8 bulbs.

Mark

Add more lights. That's what I did. Enclosing plants with a blanket or reflector could interfere with air flow and cause fungus problems.

I just typed in 140-904 @ hw.homedepot.ca and got no match. B/c of the price i typed it.