I burned a day trying to resolve this exact same problem a month ago and still have no fix. The only difference is that I'm trying to run two displays at 1400x1050 each. I've posted to a couple of different places and no one responds, so either:
1) No one is running RH9+SN41G2+TwinView
2) It's so obvious and I'm such morons that no one is willing to answer.
3) No one likes me. *sniff*
As best as I can tell, the problem has to do with memory throughput. If you check your log, you'll see:
(WW) NVIDIA(0): Insufficient memory bandwidth for MetaMode
I have dozens of those lines appear when I start up. It doesn't matter how low you crank down the displays, it'll still cause that error.
Also, I'm able to get the displays to work, albeit just barely, by adding this to the device option:
Cavaet emptor, because this display becomes extremely unstable. Be prepared to use the vulcan death grib to escape if you display goes haywire. Anyway, this disables those tests that you're getting warnings about and your second display will come up. It's just unusable.
Good luck and let me know if you make any break-throughs. I'm licked until a new driver comes out from nVidia.