IDEFix-97 supports the Tandem IDE controller, tandemat.device
http://aminet.net/disk/misc/IDEfix97.lha
You might need to remove all oktagon.device drivers before installing IDEFix-97.
I'm using IDEFix-97 on my Tandem controller with a slim IDE DVD-writer drive in my A2000. Works like a dream.
Cheers