Both of those cards will draw more power than each of those GPU power connectors on the main board are designed to handle (6 pin PSIG is only rated for 75W). And trying to draw more than would likely damage both the board (melt the Hot Air Leveled solder causing a short), and PSU. Neither is desirable.
So to prevent this from happening, you need to run an additional PSU, and route the GPU power cables into the MP via an open PCI bracket (jump the green and black wires on the mainboard connector to turn it on).
There used to be something called a JuiceBox (GPU PSU that fit in a 5.25" bay), but they've gotten hard to find (suspect they're out of production). But if you're in the US, you might be able to find one (was made under a few names), or get one used from places like eBay or Craig's List if you're interested.
EDIT: Found an
example on Amazon.
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