All of his lights are in cool tubes.. so heat is not really an issue.. but the plants do get fried at the tips if they get more than 16-18 inches tall. I would be interested to see one of these splitters in action! He is basically trying to cut down on the extra wiring that would be involved to double the amount of ballasts and fixtures he is running. And yes.. a room can be overlit.. if you have your lights in cool tubes and get them within a foot of the tops.. you will see them do some funky things indeed! It usually just causes the tip not to grow bud.. the tip stays spiny with long veggy leaves and then fattens up once the light is far enough away. Do you happen to have a link for that splitter?

Cheers!