Wow what a bugger.. so much for one 78xx card pairing with another 78xx which is what I thought I could get away with.
I just tossed out my old backup card cos it wasn't being recognised at all anymore, so I guess the new backup card will be the older 7870 instead. I was going to consider a lower-grade 79xx card but costs dictate that this isn't an option, as I need a bigger case to fit everything in right now; it's very cramped in there. That, combined with the micro-stutter issue I'm likely to encounter doesn't bode well for any Crossfire solution I had planned.
Thanks for the replies anyway guys.