Don’t know why it would be one filter, I just installed the fc8791a and it was two pieces.I did that and ordered a Fram FCF8791A from Amazon. The question and answers sections said that it came as a 2 filter package and fits a 2001 Silverado. It came as 1 filter and was WAY too big. I returned it. I think that maybe I should buy it locally so that doesn't happen again.
BTW, I used a oscillating saw to cut out the opening and it took just a few minutes and made very little mess. Made a cover out of some thin scrap steel from an old dishwasher kick plate.