The 660 manual suggests up to 16 actual feet worth of bulbs. For my 72 incher's, 2 of them would be only 12 feet.

There's a thread on ReefCentral with someone asking Andy(the Pres.) from IceCap if there were any problems adding a 3rd 72" VHO bulb to make a total of 18 feet worth of bulbs and Andy replied stating it would just add the slightest bit more heat to the ballast and shouldn't have any longterm effect.

Another person chimed in stating they had that setup for over a year so it convinced me to try it as well. ;)