Help me make my second gpu work

[eeeark1]

I now have two sapphire radeon gpu's.  A 6870 and a 5830.  I can make either one of them work by themselves but not together.  I am fairly certain it's a power supply problem but I could use some help to make sure. 

My mother board is an ASUS p7h55-m pro
PSU is EVGA 600B
I'm trying to use a powered pcie riser cable (16x to 1x) to connect the 5830 to the mobo.  The 6870 is connected to the pcie x16 slot on the mobo.
I have two monitors, one connected via dvi to the 6870 and the other connected with a vga cord to a vga/dvi adapter on the 5830.

I believe my problem is that my PSU doesnt have the connections for two video cards such as these.  I'm using the only two "PCI-E" cables from the power supply for the 6870, and atm the 5830 is only powered by the powered riser cable.  I tried using one of the PSU "PCI-E" power cables for each of the cards but that didn't seem to work (both cards seem to need both power cables).

I'm probably forgetting to mention something important but maybe someone could help anyway. 

Thanks in advance.