Ok, as you'll see from my extensive number of 'Post', total newb question.  Got my first rig up and running...
Specs:
Gigabyte R9 290 GPUs
GIGABYTE GA-990FXA-UD3 AM3+ AMD 990FX SATA 6Gb/s USB 3.0 ATX AMD Motherboard
Seasonic X-1250 ATX 12V 24PIN 1250W Active PFC 80PLUS Gold Modular

I ordered these risers:
http://www.ebay.com/itm/Powered-Riser-PCI-E-Express1X-to-1-16X-molex-Bitcoin-Litecoin-Mining-/171205234589?ssPageName=ADME:L:OC:US:3160
(Powered Riser PCI-E Express1X to 1/16X + molex)

Put the first GPU on the mobo and got it up and running.  Tonight I tried to add a second GPU with the above riser.  When I go to power on the PSU it 'trips' and provides no power.  I 'think' I have the connections correct but I'm wondering if I bought the wrong riser? In the meantime I stuck the 2nd GPU on my board just get up and running.

I read that the 1/16 riser is compatible for all cards?
I've also read several comments such as, you can't mix GPUs on a riser with a GPU on the mobo. <- is that true?

Can someone shed some light on my riser setup, I'd love to get my 3rd card setup without frying a GPU or mobo.
Thanks in advance...