How to combine folders?

[LenC]

Beginner issue, but stuck. MacOS 10.12.6. Two folders of photos, one on laptop, one the desk. I just want to combine them in one, but the two-way sync wants to delete a bunch of files. I already had to recover 1000 files from the trash due to assuming this well-reviewed software would not do something like that. This can't be hard, but I can't seem to understand how to do it.