I have tried both cards individually now and can report on the behaviour of both. Both run, and do not drop out like the SLI configuration, however while running both I get random freezes in chrome (can move mouse but the browse is unresponsive), this effects all animations and videos e.t.c. and last 2-5 seconds.
Other than that I can't report much else, they play games fine as far as I can tell, both are stable under load from furmark.
Pretty confused about this, would appreciate some advice.