I see, Mo. Due to the fragmentary character of Mozart's manuscripts, each editor has his own ideas about a workable score. LearnChoralMusic offers two versions, "Peters" and "Bärenreiter". If the one you are going to perform is neither of these and not available as a MIDI file elsewhere, you can ask a savvy friend to edit one of the files so that at least your part forms a single track.
The "Choirparts" people on YouTube have done a great job, but the method is not what I would recommend. Singing from sheet music requires reading ahead by a varying amount. Try the software mentioned, to be used while reading from the same vocal score you would have in the concert. This will also improve your ability of sight-reading; you may one day find that you no longer need emphasized MIDI, and one other day, you no longer need any computer at all for that purpose! (This crucially depends on the difficulty of the part. I confess that I still use software for difficult passages, if only to speed up the learning process.)
Software also offers the options of slowing down and transposing the music, and emphasizing your part to an arbitrary degree. If you (- anybody -) want to tell us about your experiences with various methods and programmes, I would certainly be interested. Good luck!