Generally, when someone says "If X ... but if Y" they are talking about the cases X && !Y and !X && Y. For the X && Y case you need to look at other statements. In Harris' case her other statements are that she did not consider Trump a creditable source of COVID treatment information.

Harris is saying that if Dr. Fauci says to take it she would regardless of what Trump says, and that if Trump says to take it but Dr. Fauci does not she would not take it.