Figured i'd give it a try, put each one in separately and powered it up, as well as alternating the PCI-E plugs, and always get hardware errors.
I guess this is their way of saying its over for me and GPU mining.
They've been fine during gaming, so ive no reason to believe they've had a significant failure, but clearly something isnt happy in there. Oh well, thanks for the suggestions anyway.
Run DM with -d and see which GPU is giving errors. One might actually have gone faulty. Also, are both fans functioning correctly? And are they both consistently under 85f?