The X error log may give you some more detail on why this is failing. I recommend that you try configuring each screen alone first, then worry about setting up the twinview option. I personally do not recommend using Xinerama and twinview together - I would set Xinerama to off.
I also do not recommend that you have a second screen defined. The twinview setup implicitly identifies the second screen. Adding another separate screen definition may be part of the problem.