Ah, right, now I remember why I hooked up one of my monitors to my onboard graphics...
Because, despite the fact that my graphics card has 6 ports, it has a limit on 4 outputs for no apparent reason.
Linux doesn't seem to like outputting through both the onboard and PCI graphics simultaneously, but since the odd monitor out is a TV, I'm thinking maybe I can find a way to cast media to it without it counting as a proper monitor...