How does one add a (second) support contact

[Rob Sargent]

The cluster hosting my instance needs to have an admin account to enable them to maintain our installation

[Nic Brough -Adaptavist-]

As an admin, you can add other people to the admin group(s), so you should get them an account and put it in the admin groups.  I'd recommend adding them as a technical contact on your my.atlassian account as well, if they need access to licencing or may be needed if your admin groups change.