Dual head setup - second monitor won't be recognized

[svadi]

I try to setup a big desktop dual head configuration with two monitors and two video cards.
I set up my machine with Linux Mint 13 Cinnamon 64bit.
After struggling through some problems with my ATI HD6570 video card, I finally managed to get it to work with the proprietary catalyst drivers from AMD.
The problem: I have a second ATI HD6570 installed, which isn't recognized by the driver.

I tried

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aticonfig --initial=dual-head -f
aticonfig --set-pcs-str="DDX,EnableRandR12,FALSE" 

with no results. The second monitor stays black and there is no sign of it (or it's video card) in the system, neither in the catalyst control center nor in Preferences/Monitors.

How can i tell Linux about the second video card/monitor?

[remoulder]

Perhaps you could try the suggestion in http://ubuntuforums.org/showpost.php?p= ... ostcount=2?

[svadi]

Thanks, this worked insofar as I have now my second monitor running. But things aren't satisfying, since Cinnamon went 2D, and I'm unable to configure the screens in terms of which one should be right or left. Sure, I could just swap monitor cables, but I would like to have it configurable.
In the catalyst control center these options are missing at all, and Preferences/Monitors tells me there is no RANDR extension.

I also tried

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aticonfig --swap-monitor

with following result:

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Error: option --swap-monitor is not supported when RandR 1.2 is enabled!

Eh? I thought there is no RANDR... I don't get the hang of it...
Anyway, is there something else I could do? I could get over it if there's no other way to swap monitors than swapping cables. But not having the full Cinnamon features would be annoying...

Edit: I overlooked that there is a "Graphic Cards & Monitors" subforum, so i posted my question in the general hardware support section, sorry about that. Perhaps one of the mod guys might move this thread to the appropriate place?