A small part of the Mandrake init scripts /etc/rc.d/rc.sysinit (you can append it to yours if you are missing something similar):
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# (pixel) a kind of profile for XF86Config
# if no XFree=XXX given on kernel command-line, restore XF86Config.standard
for i in XF86Config XF86Config-4; do
if [ -L "/etc/X11/$i" ]; then
XFree=`sed -n 's/.*XFree=\(\w*\).*/\1/p' /proc/cmdline`
[ -n "$XFree" ] || XFree=standard
[ -r "/etc/X11/$i.$XFree" ] && ln -sf "$i.$XFree" "/etc/X11/$i"
fi
done
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Move your XF86Config-4 file (the one for standard kernel) to XF86Config-4.standard, create a symbolic link from it to XF86Config-4, and move the XF86Config-4 file (the one for Backstreet Ruby) to XF86Config-4.bruby. For Ruby/Backstreet Ruby kernels, add to the append line in /etc/lilo.conf, or on boot prompt "XFree=bruby", leave the standard kernel as is.
Results:
Booting with "XFree=standard" or without "XFree=" (boot prompt or lilo.conf) will result in linking XF86Config-4.standard to XF86Config-4; booting with "XFree=bruby" will link XF86Config-4.bruby to XF86Config-4, so in both scenarios XFree can be started with the proper configuration file for the first X server.
And what about the other X servers?
Under a standard kernel you cannot use several independent X servers, so you should use the other XFree configuration files only under Ruby/Backstreet Ruby - there is no need for different configuration files under standard & bruby kernels.