Hi @Anonymous,
Based on the issue that you mentioned, do you want to search DisplayName and UserPrincipalName based on the search text?
Could you please share a bit more about the scenario?
As @WarrenBelz said, "you can only specify one field for searching in these controls".
As an alternative solution, you could add a Gallery and populate it will a table pulling from the Office365User.SearchUser.
I have made a test for you, please take a try.
Add a Combo Box and set the Items as below:
Office365Users.SearchUser({searchTerm:ComboBox1.SearchText})
Add a Gallery and set the Items as below:
Office365Users.SearchUser({searchTerm:ComboBox1.SearchText})
Set the Text of the Label within the Gallery as below:
ThisItem.DisplayName
ThisItem.UserPrincipalName

Hope it could help.