Thalamus allows you to swap two candidates without creating additional spots on your interview day.
Search for applicant A on the applicant dashboard
Select the 3 dots next to their name
Select "manually schedule"
In the manually schedule module, select the "force assign (override rules)" button (It will toggle blue when active) and select the calendar date you'd like to schedule into.
When given the option to open the cancelled spot on the cancelled interview date, select the option to "remove the spot when cancelling" (the second option)
Applicant A will now be scheduled into your interview day, and will display as +1 on the calendar.
Search for Applicant B and follow the same steps
View additional information about Inviting Applicants to Interview and Cancelling an Applicant Interview or Waitlist.