I asked about this when I was thinking about upgrading last year and the consensus is that each light would light up about 2 feet so that you would have a foot leftover with low light if you use 2. Mine was a 6 foot tank and I couldn't justify getting 3 and having essentially 450 w of MH on it, since it was not a high light setup. For a 60" tank, you might be able to get away with using 2 if you have it a bit higher, but then you'd have a lot of shadows I think.