SteamVR - builds from my computer complain about Vive and Monitor in separate graphics cards

When I run my Unity applications on the Vive, with the main monitor and Vive HDMI plugged into different graphics cards, SteamVR complains about the following:

"This application requires headset main monitor plugged into same graphics card"

While this happens the tracking is still working but the Vive headset only shows the loading scene.

..

One could state that the Vive requires both HDMIs on the same card, HOWEVER:

Unity builds from another company will run fine on the same setup (Vive, HDMIs on separate cards).

I have tried re-building their project on my computer, and that build still does not work. So it doesn't appear to be project-related, rather hardware related? Although why the build environment would affect the output confuses me.

In the Editor, their project also does not appear in the Vive on my computer.

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In all cases, the builds work fine on the Oculus.

From my research I have found it's best to have them on the same graphics card anyway but I was curious as to why some builds would work and some wouldn't. We are buying a DP to HDMI adapter to resolve it in the meantime.