GetCurrentSelections() will return the selections in a string, so I believe the count of that string will never be larger than 1.
You can look into the optional arguments to GetCurrentSelections() to set special delimiter for records and values, and then use a substringcount to check the number of fields you selected in:
=SubStringCount( GetCurrentSelections('\'),'\') >= 2
(I am only comparing to 2 since a delimiter is separating two fields.
Thanks very much, both answers are very helpful.
We decided to go a different way and use GetPossibleCount to limit the possible returns to 10,000. If there are more than that the user gets an error message saying 'Not enough selections. Please apply more filters.' until fewer than 10K records are returned.
That solved the speed problem.