ViparSpectra XS2000 Height and Power Setting

Hey guys I'm hoping you can help! I'm on my third grow in my 2x4 tent with two XS2000 lights. I typically hang my lights about 20 to 22 in above my plants during veg and move them closer to about 16 in during flower at 100% power. Should I only need 1 of these in my tent? Also I have seen grow diaries where they only use one of these lights and hanging at the very top of the tent with no height adjustment. They seem to do fairly well with that method. Any input would be great they appreciate it!