You can only have one person on-call at a given time on a schedule. Schedules have layers which are essentially shifts. Within a layer you can rotate shifts through a group of people and have multiple layers. Friendly reminder that the last layer in a schedule always takes precedence over the previous layer. For more information on scheduling layers, view our knowledge base article here.
In order to add more than one person on-call, you would need to do this through our escalation policy. When a PagerDuty incident is triggered, the escalation policy tied to the service in which the incident was triggered on, determines who to notify based on which user or which schedule is on the first level of the policy. You can add more than one schedule or user to an escalation policy, which allows you to notify more than one person who is on-call. For more information on escalation policies, view our knowledge base article here.