DAX to filter multiple values in one column

Try:

A & B =

VAR A = CALCULATATBLE(VALUES(Sheet1[User]), Sheet1[Certification] = "Cert A")

VAR B = CALCULATATBLE(VALUES(Sheet1[User]), Sheet1[Certification] = "Cert B") 

RETURN

COUNTROWS(INTERSECT(A , B))

use the measure in a visual as a filter in the filter pane an set the value to 1