You could always put the second PSU on top of the case or something
It'd be cheaper to run on a pair of PSU loaded under 70% than a single big one loaded up at 90%. The higher the average load, the faster the PSU will die and therefore cost you more per bitcoin due to the replacement cost. Also PSU tend to be most efficient at around 50% load so you'll save another few % off your electrical costs.
I am really short on space - even the top of my case is taken - joystick, lamp, external HDs lol Besides, i'll only be maxing out the GPUs when I sleep. As I work from home, this is also my work PC and I mine whilst working but at a setting that doesn't tax the GPU's. I don't pay the electricity bills so its not such an issue for me.
I wish I had the space to leave this all caseless and out in the open!