Actually, two cards can be run. Some forum members have done it. Also, that 300w maximum refers to what is provided by the PCI-express slots (not power taps). Your Mac Pro's power supply can deliver a kilowatt of juice. People here have either used a spare molex connector from the optical drive bay (in 08 Mac Pros) or by using a Y-splitter on the PCIe power tap on the logic board.
However, a search of 's website would have shown you that it's putting the cart before the horse.
There's no mention of SLI or Crossfire anywhere on 's website, nor any mention of using more than one GPU for anything but display output. There is no support in OS X at this time for using more than one GPU simultaneously for anything other than display output.
The reason people have two GT120s is not for "putting them in parallel" but to have four different display outputs or the ports you need.
Realistically, having more GPUs is not much more helpful than having more CPUs: software needs to be coded to take advantage of it. And that's only just been implemented in Snow Leopard, so now people need to write the apps.