You can do this, but you need a "y" cable for the 24 pin connector, so that it takes two 24pin headers, one from each PSU and goes into the one connector on the mobo. I think the silverstone TJ07 comes with one, but they can be obtained from the usual suspects.
You may want to consider an FSP booster instead. Or you can run a PSU always on with a 24pin jumper cap (i.e. an unbent paperclip: google PSU jump start or some variation). I run an FSP booster for my second rig, because it has a 400W PSU, and needed an extra bit of juice for a 280GTX.
I also run another PSU just for fans and testing outside my rigs with the paperclip method. The PSU won't shut off if you use the jumper method, but with the y cable it will power off with the PC.
It won't put out much juice if the gpus don't need it, but it won't switch off.
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