All six monitors would have to be connected to the primary gpu else eyefinity won't work (you can only get a really big expanded desktop if you plug them in both). So the primary card would need to have at least two display ports and two relatively expensive MST hubs to allow three monitors per display port (or DVI-D + DVI-D / DVI-D + HDMI and 2x two output MST hubs). And finally.. I don't think 2x R9 270x's will be enough to drive a game at 5760x2160, at least not on high settings, for better experience, a R9 290x (or two) would be good.
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