Two Xserve and Xserve RAID configuration advice

I'm configuring a brand new set-up and I'm looking for input in RAID configuration.
Basically I have two essentially identical Xserves: 2.3x2, 80x2, 500 and a fully loaded XRAID running 6 drive RAID 5 with a hot spare on each side all running through a fibre channel switch. One Xserve is slated to be a pure fileserver and the primary XRAID access box and the other will handle web/email/database serving etc.
I'm going to use the first 80 as a system drive (mirrored to the other 80 for redundancy) and I wanted to carve out about a 500gig partition of one of the RAIDs for the webmail server to use as its data drive so it would have the RAID 5 protection. Unfortunately to properly get this configuration I think I'd need to heavily slice one of the RAIDs and then use LUN masking to hide the drives from the respective machines. I guess I could then use Disk Utility to to concatenate the remaining slices back into a single volume - but this seems like adding layers of complexity to what seems like it should be a simple task.
Another thought I had was to re-create the RAID of one side as a 5 disk array, use the extra 500 gig drive as a mirroring drive for the internal Xserve 500 and while I couldn't swap them out directly I could replace a 500 (if it went bad) and use the mirror to bring it back to an immediate pre-failure state relatively quickly.
At this point I'd settle for mirroring one Xserve 500 to the other Xserve 500 which would give me a direct swap-out replacement - but I don't know if that's even technically possible. It has gotten complicated enough that I'm considering simply using the internal 500 as the data drive and depending on heavy back-ups for protection.
I'm looking for any advice, thoughts or technical tips eveyone might have to offer.
TIA,
=Tod

I must not have explained what I want to do clearly enough. The complication is that I want Xserve 1 to see Array 1 and 80% of Array 2 and Xserve 2 to see the remaining 20% of Array 2, but neither must be able to see the same section at the same time or very bad things can happen.
The only way I'm aware of to do this is to slice all of Array 2 into 20% slices and use LUN masking to make 4 slices visible to Xserve 1 and the 5th slice visible only to Xserve 2. I could partition Array 2 into 80% and 20% partitions and then unmount the 80 partition from the Xserve 2 and unmount the 20 from the Xserve 1 using disk utility. But the potential for something bad happening down the road exists unless I can find a way to suppress volumes from mounting. (I saw this hack: http://www.macgeekery.com/gspot/2005-12/hiding_partitions but need to investigate much further before actually trying it.)
Also the back-up unit is tied into the switch and therefore everything needs to run through the switch.
Thanks,
=Tod

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