What K factor should be used if two players are in different K factor brackets? Apparently, when calculating a player's USCF rating difference after a win or loss, the K factor that is used is based on the player's rating. Now, I was always under the impression that two players with ratings a and b would play each other and that a difference in score is awarded to both of them such that their ratings after the match would be a+d and b-d (assuming player A won the match).

But let's say you have a player at 2075 (who would have a K factor of 32) and another player at 2125 (who would have a K factor of 24). Which K factor would be used? I could see a case for either of 32, 24, or even 28 being used in this case. What does USCF do here?
K-factor is used for each player individually. So, if one of them got 30 K-factor, his rating will change on the basis of that K-factor, while another player with 10 K-factor, for instance, will get rating on the basis of his own K-factor. 

There are Chess Rating Calculators on FIDE's official website. You can have a look: FIDE Rating Change Calculators