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SLI rig won't boot with CSM disabled

Level 7
I have an ASUS Sabertooth 990FX/Gen3 R2.0 motherboard and two GTX 970 video cards (made by MSI). Both video cards have UEFI GOP VBIOSes and individually, they each work perfectly when I disable CSM - pure UEFI booting happens normally.

When I install both cards and disable CSM, I get a black screen when I start the machine; it does not POST. If I use the jumper switch to restore the motherboard's default firmware settings, the machine will again boot normally. Similarly, if I remove one of the video cards (whichever one is installed as secondary at the time), the machine will boot normally, even if CSM is still disabled.

I had the exact same problem when I tested a pair of Radeon cards (identical to each other and both had UEFI VBIOSes). Each Radeon card worked perfectly when installed individually, even when CSM was disabled. It is as if disabling CSM precludes the use of multi-card setups.

I assume there is a setting I have missed because a motherboard that is advertised to support both pure UEFI booting and SLI/Crossfire should be do so simultaneously. Pure UEFI booting (i.e. with CSM disabled) is potentially advantageous for security, and while SLI is not as popular as it used to be, I have two identical cards and I might as well use them both, considering the ample size of my chassis and PSU.

Any help is appreciated.