I've run a 600w MH side by side with a 600w HPS & it works fine. I thought I remember seeing something in an Ed Rosenthal book that the HPS drowns out the light of MH but I cannot remember the certainty on that. I do remember reading that if the MH sits behind glass (like mine does) it takes the benefits of growing under MH away so now I use 2 HPS lights side by side instead and they work out just fine.