What did you use to hang the light fixture? What type of bulbs are you using?
Lucky for me, hanging was pretty straight forward because I live in a 120 year old factory that was converted into apartments. So, I spent $5 on some clips for drop ceiling frames and 10 feet of black chain at Lowes. I shot this with the flash so you could see it clearly, but in reality it's toned way down in the room and you don't notice it so much. Eventually I'll get around to splicing the wiring together and removing the plug on the end of that extension cord and tidy it up a bit. It's all thrown together really. I didn't want to commit to it until I was sure the plants would like it. So far I'm very happy, and the plants seem to be too... Bulbs are "4 for $6" bulk no brands from Lowes. "Daylight" color temp, which I think is 5K if I remember correctly. Nothing fancy I assure you. I also dry dose ferts, and micros come from Flourish Comprehensive. I run pressurized CO2 through the end of a chopstick into the intake of an FX5 canister. It's a 40 gallon breeder.
I tried Alum. foil and I would say, the foil bag I found inside Kellogs has the best strikes I've tried.
Here's another shot. I tried getting a pics with lights on but were not good photos. There's just too much light.
This shot has more reflection from the camera.
Its not the same from the photo above.
One of the things about fresh water planted tanks that seduced me into keeping aquariums again after many years away from the hobby is the fact that they could be as simple or complex as you want to make them. Unlike reef keeping, where you can drop $20k without blinking. So experiment away!
...well, just looking at the size of the tank would make me think that 2 fixtures might provide enough light.
Trial and error is likely necessary... in any event, the whole project is low cost, and probably fun at the same time.
Don't forget you have your lamps sitting right on the glass. The height of the light above the water will make a difference in the overlap. I don't see any areas in my "18 inches front-to-back" 40b that are lighter or darker. The length of my tank is the same as yours (36"). I bet if you lifted your lamps you'd see a big difference.
I just checked and the bottom of the CFL is 8.5" from the water surface. The bottom edge of the shade is 4" from the water surface. So although these shades are deeper than the shades you're using, the spread is much better at the height they're at.
Also be aware that Fluker make these same lamps, but the inside of the shade is white. I chose the Zoo Meds specifically because they were aluminum inside. Just as a FYI, I'm using 15w 6500K daylights in those. They're generic Lowes house brand. Months ago when I was running DIY CO2 I tried 23w "natural light" daylight bulbs and it was way too much light. I was combating every algae known to man. These bulbs seem to have all the PAR they need to get the job done.
So jart, the take-away of all this is that "it depends"