DA was designed for a Reseller having full control over that particular IP and not letting other Resellers having access to it.
You can set multiple resellers to use it, you'd just need to add it to that particular Resellers /usr/local/directadmin/data/users/reseller/ip.list file after he's created. Once it's set to shared, it's shared for any reseller with it in his ip.list file.
John