Okay I fixed it finally. After much messing around with settings, new drivers, etc, none of that worked.
The 2nd display wasn't showing when I booted the machine up but I knew it should be. I can't be certain what exactly I did but try any / all of these:
1. Re-seat the video card into the PCI slot.
2. Change your primary display to the second output on your card (if you have one)
I put it all back together, booted and immediately the slave display showed the boot message. The "unable to save display settings" error message is misleading because it implies that your problem is likely to be software, where as for me, it was hardware. Lots of threads on the internet about this. Fiddle with your hardware until you get your displays showing the boot screens.