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Ok so, not crossfired in a really long time. how come my second card turns itself off?
Do you have MSI afteburner installed? If not download this tool, its a great tool btw.
Go into settings and disable ULPS. If you hover over this setting it should tell you what it does. In short it disables Ultra low power state which stops the secondary crossfire card going into a idle sort of state.
It does warn it can cause instability issues but i think this is something rare but its worth a shot to see if it stops your problem. Also you have to disabled ULPS every time you install the graphics driver i believe.
I have had AB do this after a driver update, usually a reinstallation ( of AB ) sorts it.