Crossfire finally works on my 4870x2 
Unigine Tropics got a 10 FPS increase.
I got some error messages in dmesg though:
Code:
[ 2911.922373] [fglrx:firegl_find_any_map] *ERROR* Invalid map handle!<3>[fglrx:drm_vm_open] *ERROR* map not found -> inconsistent kernel data!!! vma_start:0x7f80d3652000,handle:0xd0000000
[ 2912.198467] [fglrx:firegl_find_any_map] *ERROR* Invalid map handle!<3>[fglrx:drm_vm_open] *ERROR* map not found -> inconsistent kernel data!!! vma_start:0x7f80d2cc5000,handle:0xd1068000
.....
Gentoo Linux with kernel 2.6.30-r4
ATI Catalyst 9.7 (fglrx 8.63.2)
xorg-server 1.6.2.901