From aplawrence.com:
OSR5 FAQ
Can I have both IDE and SCSI Drives?

Mixing SCSI and IDE drives can be a problem if you want the root disk to be scsi. The short answer is that you can if your BIOS lets you select which drive to boot (if not, you would have to boot from a floppy). Assuming everything else is in place, "defbootstr hd=Sdsk" should work. See SCSI for a full discussion of this.

Take a look at chapter 14 on aplawrence.com - you have to do some reading to understand how to get the drives to work - there is no simple answer.