You could make use of the Automations in Freshdesk, such as Ticket Creation, Time Triggers, and Ticket Updates to send escalation emails to multiple agents. Using automation, you could send an email to an Agent or a Group if a certain condition is met. 

An example would be to set up a supervisor rule(under Admin > Workflows > Automations > Time Triggers > New Rule) with 

Condition: "Hours since first response due/overdue greater than" (depending on your use-case) 

Action: "Send Email to Agent/Group" add the agent's name or the group's name to whom you would like to escalate.

Summary of the sample rule:
Sending escalation email to multiple agents/groups in Freshdesk