when the secondary dns is used?
is it used when the primary is unavailable?

Each DNS domain has a primary nameserver, which contains authoritative
information regarding Internet entities within that domain. At least
two authoritative name servers are required for each DNS domain, so at
least one secondary nameserver must be configured.

The secondary nameservers periodically download or copy authoritative
information for the domain from the primary server and supply
authoritative answers when queried about that domain.  There is a time
limit on how long the secondary nameserver can continue answering
queries when it cannot contact the primary.

Apart from the fact that a secondary nameserver automatically copies
its information from the primary, the DNS does not differentiate
between primary and secondary nameservers.  They are used more or less
equally whenever a domain name lookup is performed.

Your right

The secondary DNS is only ever used if the Primary is down.

The importance of a DNS server to do most anything on the internet
nessistates a secondary or backup DNS server.

Try this, enter only the primary and everything will also work fine,
unless of course the primary one goes down.