Slackware ATI fglrx radeon libs
 

For compiling and trying the open source drivers for ati, they load the files at /usr/local.

The ATI installer loads fglrx and its libs... at /lib
Normal slackware loads radeonhd and ati at /usr/lib/xorg/modules/drivers

So far I have been using mod'd slackbuild scripts and installing at /usr but maintaining is a getting tiring with each git change.

So the question is, if I have the same named file in two different locations that are seen by the system, will X know how to choose the right one?

you can install only one version of fglrx at a time

man xorg.conf