I would like to add (2) 250w metal halides to my 90G along with (2)110w VHOs, and I have 2 choices, use a single (dual) ballast for both, or buy (2) seperate ballasts (one for each light). What is the preferred method? I have plenty of outlets, and plenty of room for either configuration. Is it less electricity for one unit or two? Is there an advantage to having two units vs one? It seems to be cheaper to buy the one (dual) unit.