Hps lights have the needed yellow and red spectrum at a higher output than most led lighting or metal halide bulbs. This means hps lighting will stimulate flower and fruit production more effectively.
I guess you googled some HPS advert..
Will an extra 600w hps increase yield.. yes , in theory, if in practice you can keep them far enough away that radiant heat isn't to bad, should be possible.. but the big one is can you manage the air temp..?
If your room is 70f lights off and 80f lights on with 1 hps, it's going to be 90f with a 2nd one.
Unless you've already got oversized extraction to handle doubling the extraction rate then you'll need to buy new fans.
One option would be air cooled hoods.. in this particular situation cooltubes would be useful due to the narrow reflectors and minimal additional resistanceto airflow, 2x large hoods would be a struggle to fit in a 4x4.
There's a lot to think about.. a 1000w in an xl hood would give better coverage..
Most people can't manage that kind of heat in a tent.. but some can