MX preference is an ordering hint, not a load-balancing percentage. A sending MTA normally tries the lowest preference value first and moves to other eligible exchangers when delivery cannot proceed.
example.com. IN MX 10 mx1.example.net.
example.com. IN MX 10 mx2.example.net.
example.com. IN MX 20 mx-backup.example.net.Hosts at the same preference are equivalent choices. The higher-numbered host is lower priority.
Backup MX is still production
A backup MX that accepts mail must be secured and filtered like the primary path. Attackers can intentionally connect to a weaker backup host. It also needs a reliable way to deliver queued messages onward without becoming an open relay.