Two Different SCSI controller’s fighting

I have PII-400 Server w 256 megs of RAM and an Itel Chipset on the Mboard. It has a Mylex FAST & Wide SCSI controller with a raid 5 array of 9Mb drives (18 GB total).
I am trying to add a adaptec 2940 narrow SCSI2 contrllor to this config. When I add the second controler the SCSI ID’s on cards go nuts…. sometimes the mylex is controller 0 and the adaptec is 3 or when you reset the adaptec is 0 and the mylex is 1. This of course reeks havoc with the tape drive I trying to put on the adaptec controller. When the SCSI bus resets it can’t find the drive anymore…no tape backup for me!! BOSS unhappy HELP!!