I dont think you need the dummy plug for the 5970. In GUIMiner, have you select the different in each tab ?
One should read
I have feeling you're using the same core on both tabs
good question, i think its set up ok, i have 3 tabs 0-0 which is 5870 then 0-1 and 0-2 which i think is the 2gpu's on the 5970 all cypress, my cpu is listed but i'm not touching that, i do beleive i am using one core though as when one starts at 10-18mhs the other drops by that amount. i am also asking in the tech support section, just going through driver versions atm