Another fixture would improve coverage but isn't needed. If you want a carpet and tall plants behind then the carpet would be shaded if you only had one fixture for instance. Or try raising your lighting, I have to have my LED 16" off the top of the tank so the top corners aren't shadowed. Fluorescent has wider beam angle so probably wouldn't need to be that high off the tank though.
I put a 2 bulb T12, not even a T8 on the tank, just to see how it looked and my plants were pearling down to 12" so I am sure the lowest plants were getting plenty of light in my 24" deep tank. I would put 2 full length fixtures on if I was planning on using such dinosaur lighting.
You are likely to have more success with less light than more! If not using CO2 then less than 30PAR, if using you will be in trouble over 100PAR and 50-75PAR is fine for most plants.
See the sticky thread in the Lighting forum for information on how much PAR you may be getting now. The efficiency of the reflector is extremely important. An AquaLife would be fine even if 35" off the substrate as would an ATI but even on top of the tank a Coralife would only provide low light.