You can't! Be careful with that. This sort of environment (with clustered filesystems) is what CLVM was designed for.
If you're going to use LVM2 individually on each server, there is a crucial rule of thumb to follow: any updates to LVM2 configurations (that involve the LUNs) must be immediately followed by a reboot of each node. I wouldn't be any less cautious than that, personally.
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edit: Here is a good thread on the topic -
http://www.mail-archive.com/ocfs2-us.../msg01099.html
