I also use 2x 600w hps fixtures. You can get away with 9 plants under one, if the plants are medium size, grown in 3 gallon pots etc.
I'm currently only using one of them in a 4x4 tent and it's great for that. A 4x4 tent is 16 sqft and double that gives you 32 sqft so you are correct to use 2...