If I set up gmirror for a SLICE, how do I set up the MBRs to both be bootable?

[Rudy Rucker]

I have two disks, ad0 and ad8.
  ad0s1 and ad8s1 are a gmirror
  ad0s2 and ad8s2 are in a zpool

Other than pulling ad0 and rebooting, is there a set of steps I can take 
to make sure ad8 is bootable?  Is an identical MBR a good enough test?
  dd if=/dev/ad8 of=ad8.mbr bs=512 count=1
  dd if=/dev/ad0 of=ad0.mbr bs=512 count=1
and here are the md5's of the MBR files
  MD5 (ad0.mbr) = f7dced6b4ab7f171cc66ae44ece16a73
  MD5 (ad8.mbr) = f7dced6b4ab7f171cc66ae44ece16a73
I want to check some production boxes to make sure they are set up 
correctly.  I've never had a problem with using whole disks in gmirror, 
but I am new to using gmirror for slices.


Disk setup:

 > gmirror status
         Name    Status  Components
mirror/gm0s1  COMPLETE  ad0s1
                         ad8
 > zpool status
   pool: tank
  state: ONLINE
  scrub: none requested
config:

         NAME        STATE     READ WRITE CKSUM
         tank        ONLINE       0     0     0
           mirror    ONLINE       0     0     0
             ad0s2   ONLINE       0     0     0
             ad8s2   ONLINE       0     0     0

- Rudy