Extra motherboard power is really only needed when overclocking or perhaps when multiple PCIE devices are trying to draw a lot of power from the slots. It's not required for a normal configuration and wouldn't be the cause of memory incompatibility.

Have you tried setting a lower memory speed before installing the 2nd module? You may not be able to run the kit at full advertised speed but getting it running at any speed confirms that the modules and the motherboard are working. Otherwise you need to try different modules in different slots to see if the issue is a certain module or motherboard slot.