The question is: is anybody using OS4 with A1200 and with HD directly connected with the mobo (and with two units attached to the cable)? And with IDEFix (or similar HW)?
not sure what you mean.. but I did use IDE-Fix Express and OS4, though the drive was obviously connected to the Express, and not the mobo.
I documented how I installed OS4, perhaps it helps you:
boot from os4 disk
choose english language, netherlands (I dont recommend this

), UTC, american keyboard
start installation from os4 desktop
choose a1200, bvision, bootvga
max resolution set to 1024x768
choose idefix, floppyfix
choose base page access, disable grexfix
edit os4:kickstart/kicklayout-a1200 remove semicolon (

at the end of the line regarding 'bootdevice module'
put file in kickstart folder called 'BootDevice' with just 'hd2' without quotes *
do a warm boot
* my OS4 drive is called like this in HDToolBox
BTW Im using SFS (68k) for my drive.
AND dont forget the OS4 update: AmigaOS_4.0_Feburary2008_update.lha