From one system you will need to export the /usr filesystem and on the other system you will need to mount the remote /usr using NFS.
To export the filesystem you will need to make an entry in the /etc/exports file and then run the /usr/sbin/exportfs program to allow it to be accessed.
Once the /usr has been exported you can use mount to mount it into the local filesystem. If you have systema exporting it's /usr and systemb is to mount it then
/etc/exports would have an entry like this
would make the /usr available to systemb so
mount systema:/usr /mnt
would mount /usr into the systemb filesystem at /mnt
If you want the systema:/usr filesystem to be mounted at boot then you'll need to add an entry to /etc/fstab e.g.
systema:/usr /mnt nfs defaults 0 0