Are you sure both GPUs are running with SS 6.4, Ferdiad? Well, I am using two 6970s in Xfire mode and the secondary GPU will not run. I'm not sure why yours could work, maybe you did some modifications to it or simply because it's a dual GPU card, maybe even something else.
I did alittle more digging and I found the solution for my Xfire and msi on screen display. It turns out you have to disable the border-less window in the default.cfg for SS6.4.
I think it has something to do with crossfire not working in windowed/border-less mode. I understand SS 6.4 enables border-less mode to switch in and out during the changing turns.
Found the info over here: http://hardforum.com/showthread.php?t=1633731
This is what I changed to get my xfire working properly, from the default.cfg in ss6.3 folder.
Cheers.