first, i'm assuming that your OPTIONS weren't commented out when you tried using this file. but i would say add OPTION "Xinerama" that made it work for me, also your XF86Config file looks somewhat different from mine, for instance, you don't even have a screen section. attached is my XF86Config for dual screens. lemme know if it works out.
ps, if you ever come across a way to change the splashscreen(not remove it) let me know