If you use one server as (only) storage you may or may not distribute the load (speed of network transfer vs. local storage) but double your outage risk - Nextcloud is broken in each of the servers fails.
There might be reason to split Nextcloud installation over two servers I would rather suggest running Nextcloud on one server and user the second as backup destination. Depending on your need you might have the second server just store your backups or become a cold standby (by doubling all the installation steps) so you could easily swap in case of failure…
Thanks for your reply. I have problem with the limit of the storage capacity in the server where Nextcloud server installed.
What about installing nextcloud server on the NAS server itself where all files going to be stored? I am planing to buy Synology NAS by the way.