Two things to point out:
1) There is a challenge associated with spending time as either Gat or Kinzie, so you don't have to worry if you think you're spending too much time with either of them. To make your tracking easier, you can bring up the challenge to help you know how long you've got left before the limit is over.
2) To switch from one character to the other, return to the Ultor lobby in the southeast quadrant of New Hell (has its own map icon, hard to miss) and change places.
You will end up needing to go back and forth between the two to collect the commentary logs for both characters as well. You will only find Kinzie's logs as Kinzie, and so-on for Gat's. In clearing each district of its collectibles, you will find that the two hours will just fly on by!