A SCSI bus must always be terminated on both ends. With internal devices only, the Supra Drive and the last drive on the cable require termination.
If you don't have / can't find the resistors packs for the Supra you can alternatively use an external 25-pin terminator (Apple style, an active one is a good choice).
A badly terminated bus may not run at all or may cease to run at any time.
If both devices are internal and you have no means of terminating the controller but there's no problem with the other two, you can plug the controller in the middle of your SCSI cable and put the devices on its ends, terminating it properly. It'll work but don't think about using the external SCSI connector like this - that would illegally branch off a third end of the bus which will never work.
