If you have 2 or more users that rotate primary and secondary on-call shifts, then you will want to create two on-call schedules and add each of those schedules to a separate level of an escalation policy.
Schedule example articles:
This schedule has 2 users on call. When User is the first point of escalation, User 2 is the backup, receiving all incidents that User 1 does not respond to. When User 2 is the first point of escalation, User 1 is the backup, receiving all incidents that User 2 does not respond to.
You will need to create 2 on call schedules. One for the escalation policy level 1, and the another for escalation policy level 2.
Go to Configuration → Escalation Policies and either create a New Escalation Policy, or edit an existing one. Add Schedule 1 to Level 1 of the escalation policy. Then add Schedule 2 to Level 2 of the escalation policy.