The simplest way to kick off problms with /usr merge is to *not*
make /usr a mountpoint. There is no utility in splitting the OS in
several partitions. This wouldn't help you recover the OS if one
partition is broken since the package manager installs/deinstalls files
not only on /usr, but also /, /etc, /boot, /var. Splitting is even
discouraged if you use a filesystem with snapshots like btrfs (whih
isn't my case). The only thing which may (and should) be on a different
disk (not partition) is the precious user data, ie /home.

--     Didier