I ran my 60 gal (120 x 45 x 50 cm) with 2 40W tubes for years getting excellent plant growth. At first it was with the Phillips New Generation Flutone (which are like natural daylight) and later, because I was cheap, with plain cool whites. Plants grew well. My pygmy chain swords at a depth of 45 cm were nice and red and running rampant.I have several tanks with 4' long fluorescent lights.
(2) 40 watt T-12 over a 55 is not really very much light. With REALLY GREAT reflectors you might be OK...
As you are using Gro-lux tubes which peak in the red you should do even better. I have 3x 30W tubes over the same tank at this moment: 1 Grolux and 2 Aquastars. The HM growing under the Grolux grows horizontal while those directly under the Aquastars are growing straight up in search of more light. Under the Grolux and at depth of 45 cm the Riccia pearls. I have Cabomba in the tank and it grows like a weed.
Instead of the 2nd Grolux, rather go with a Triton, GE Aqua Rays or something which is more full spectrum (like Biolux) or like daylight so you can balance out the Grolux which look pale and look more red and blue rather than green.
Best of luck HVS.