How do I set up a seasonal employee for STP? In this instance, they only work two months in a year

If you started processing STP mid-year and you had seasonal workers who worked prior to this you can run an update event to send the year to date information to the ATO- https://quickbooks.intuit.com/community/Payroll-and-STP/Create-and-lodge-an-update-pay-event/m-p/275360

You can also apply for a deferral via the ATO Portal: https://www.ato.gov.au/Business/Single-Touch-Payroll/Get-ready-for-Single-Touch-Payroll/Single-Touch-Payroll-deferrals/ or seek assistance from your Registered Agent