ok, got it... the resolution must go in the metamodes of twinview.
something like that:
Option "MetaModes" "1280x1024 , 1024x768; 640x480, null; 800x600, null"
meaning, go to 1280x1024 (1st screen) 1024x768 (2nd)
when changing to 640x480 (1st) null (2nd, don't use)
hope i helped someone