I can't do exactly what you require but I can add an interactive message into the form when the fields match.
I created a custom field called "mismatch". See attached screenshots for changes from the default values.
I added this field to the form and then amended the initial values of both Sponsor Name and Visitor Name so they had differing values.
Now when the form is completed, if the Sponsor Name and Visitor Name fields match, an additional field is displayed with a warning. See screenshots for the same values and differing values.
This doesn't stop the form being submitted but does give the end user a warning message.