You can certainly run Crossfire together with a R9-280X and a HD 7970.
The number of outputs on the card doesn't matter. You can only using the display outputs of one card anyways - The primary card (i.e. tpyically the card in the top-most PCI-E X16 slot).
In Crossfire Mode, all your displays needs to be connected to the primary graphics card. The secondary card does not display anything, and should not have any of your displays / monitor hooked up to it.
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