oh god im not sure to be honest
have you tried moving the 8inch lcd so you have both of them on the same card ?
ps: found this quote in a review
"If a hub isn’t available and daisy chaining not feasible, the 5 ports can still support up to 4 displays in Eyefinity as illustrated at the back of the box."
ok this is weird according to amd 5 monitors are not supported but 6 monitors are ???
http://support.amd.com/us/kbarticles/Pages/gpu50-ati-eyefinity-display-configs.aspx
That's old information, it only took like a year of badgering to get support. 5x1 is now supported, albeit only on the 5870 Eyefinity 6 card. But when the HUBs are released, the other 6000 series cards will get support. the 6990 also has 4 mini DP and 1 Dual-link DVI, so it support 5x1 (but isn't released *le sigh*).