MX records and relayhost: Am I going this correctly ?

[White, Daniel E. (GSFC-770.0)[AEGIS]]

MX Records: 

        mydomain.tld.   33      IN      MX      10 relay-1.mydomain.tld.
        mydomain.tld.   33      IN      MX      20 relay-2.mydomain.tld.

with relay-1 and relay-2 having appropriate A and PTR records.

Then in /etc/postfix/main.cf, I have 

       relayhost = mydomain.tld

I have seen this work with relay-1 unavailable.
The outgoing message sat in the queue for a brief time, then went out through 
relay-2.

I found a Red Hat solution https://access.redhat.com/solutions/4025391
that suggests using 

        relayhost = [relay-1.mydomain.tld]
        smtp_fallback_relay = [relay-2.mydomain.tld]

Comments ?
Suggestions ?