I'm wondering what would happen if I use 16/2 and 8/2 cotton alternating on the same warp?

Hi Karen, you would have to adjust your ends per inch (EPI) in each section of 16/2 or 8/2. If you are alternating the two yarns - assuming that you were putting 2 - 16/2 threads between your single thread of 8/2, you would use the same sett used for 8/2. Have you watched the video - Let's Have a Little Chat about Sett - Season 1 Episode 4 and All About Yarn - Season 1 Episode 7? Jane's Master Sett Chart can be found as a PDF in Episode 4.

Thanks