If I split the data from one project into two papers, does the second paper need to mention the first?

I recommend at least citing it. In the methods, you can also explain that the data come from a larger project.

Write each paper's argument following its own logic, and avoid copying and pasting as much as possible, so that similarity-checking software does not flag a high overlap and you are not accused of self-plagiarism.

Splitting a project into papers takes skill. If the two papers are too similar, and the data could clearly have been published as one paper, you will be criticized for salami slicing. The two papers should have different main questions and arguments.

• Multiple Publications From the Same Dataset: Is It Acceptable? https://www.ncbi.nlm.nih.gov/pmc/articles/PMC8880828/

For example, if you publish two papers from the same study, one on WBC and one on CRP, both are inflammatory markers, and you will be criticized.

But if one paper from the same study analyzes survival and the other analyzes improvement in pain scores, that is acceptable.