4 GPU Mining Rig With Dual PSU

Hey guys im currently working on my first mining rig for ethereum.
This is my hardware:

Mainboard: MSI H87 G43
CPU: Xeon 1230 V3
RAM: 8 GB
GPU: 2x R9 290X + 2x R9 290
PSU: 1x Corsair 850 Watt + 1x be!quiet 550 Watt

Couple of days ago I've read something about a dual PSU instead of one big psu like 1200 Watt.
I got the second one (550 Watt) for free and bought the 2 gpus R9 290 'used' for each 110€. So i think its okay that i dont have the best price/power GPU (like Rx 470 or 480).

I also bought 2x USB Riser i found in a thread here. The only thing i dont get is, how to turn on both psu?
I got my two R9 290X plugged directly onto the mainboard with power cable to the 850 Watt psu. And the other ones R9 290 via usb riser to the mainboard and power cable to the other psu (550 watt). If i turn on my pc, only the gpus connected to the mainboard are starting. How do i turn on the second gpu?

Maybe the 850 Watt psu is enough for my 4 gpus? what do you guys think?

Thanks for the help