I had a similar problem with my dual 5970 installation (but in contrast to your problem, it worked fine with a single card). It turned out to be ATI Catalyst bug and upgrading to 10.12 solved the problems.
Could you try to log into console while booting? Ctrl-ALT-F1 should give a login screen. You may also try installing openssh-server and login remotely (actually I had to do this way because console did not work) but it maybe hard when there is no full system.
You may indeed try installing the system with one card and try installing the drivers later. There is also possibility of a motherboard-card conflict. Does it work with Windows?
Thanks for the quick response.
I am actually installing using the in-built graphics card and am going to try and install the 5970 later. Do you think this will work?