I am pretty sure the dual you can run both at the same time. For a bigger setup it would be nice to have, or even if you wanted to do that in the future. I think you can also run MH in one and HPS in the other. I just bought a lumatek 600 W (single) with a digilux bulb. I am satisfied for the moment but I have been using it for about 2 weeks. I was using an 8 bulb t5 to flower. So this is a huge step up. The heat is an issue but I have a wing reflector and will be upgrading to a hood at some point soon.
I think you said you were having issues with height right? I would really take that into consideration because the hps will need to be much higher than your LED's and such.