The controller, or "initiator" in SCSI-speak, is assigned a SCSI ID just like every other device in the chain. It is virtually always ID 7. However, you can get your controller to have a different SCSI ID. If you do that, you can have one controller at ID 7 and another at another ID on the same chain. As long as they don't both try to control the devices at the same time, you should be okay. That said, there will still be some downtime in the event of a changeover. In a catastrophic failover, the new machine will at the very least have to repair the filesystems left damaged by the sudden shutdown of the other machine, if not repair the Exchange store. It might even have to reset the whole SCSI chain. It's still better than having to move cables manually, though. Even if it's a scheduled changeover, the filesystems will have to be sync'd on the primary machine before the other machine can take over, and a reset might have to be performed there, too.

It's a pertty unstable configuration, but many people have put it into production. You have to be very, very, careful about making sure both computers don't access the machine at the same time. This is more difficult than it sounds. You wouldn't want a simple network failure to cause both machines to think the other is down and cause them both to start writing. Maybe folks have implemented a check on the SCSI level these days.

So, to sum up, it's electrically possible and protocol-possible, but, I think, only as a side effect of the specfication, not as a design point. In addition, it's much harder under Windows, where the OS wants to grab every storage device it can see right off the bat. But it's doable.