The problem is a little hard to specifiy. Under windows, folding informs me that the secondary card is not supported at this time, and suggests I run the cpu client instead. If the primary card is removed and monitor moved over, it then behaves fine.
Under linux, I get 'unstable machine' along with '.. this is a sign of a more serious error, shutting down'
That is a somewhat incoherent thread. If anything, it suggests that I should have gone with the 4870 rather than the 280gtx. A useful reference though, thank you.
Right now I'm trying to cool down my 280 by underclocking it, with bugger all success. Will give putting both cards in one machine another go in the near future though