DAX to filter multiple values in one column

Try:

Table = SUMMARIZE(filter('Sheet1', Sheet1[Certification] in {"Cert A"} && Sheet1[Certification] in {"Cert B"}), 'Sheet1'[User],'Sheet1'[company],'Sheet1'[Certification])
)

 

Though I'm not sure if that is what you are after. The && in the filter means the values should be Cert A and Cert B, as opposed to Cert A or Cert B

If it's OR you are after, try:

Table = SUMMARIZE(filter('Sheet1', Sheet1[Certification] in {"Cert A", "Cert B"}), 'Sheet1'[User],'Sheet1'[company],'Sheet1'[Certification])
)

 

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