https://bugs.freedesktop.org/show_bug.cgi?id=29217
--- Comment #12 from boris64 bugs.xorg@boris64.net 2010-07-24 05:14:12 PDT --- Update: Enabling "Repost Video On S3 Resume" in BIOS seems to help a lot. I've send my computer into sleepmode for a couple of times now and the second monitor has always been working fine after resuming.
I actually dunno what this BIOS option does, but it seems to be a solution.