How should I interpret a suggestion to "synthesize" two sections of a paper?

Synthesize in this context means to merge the sections into a coherent whole. So the reviewer is probably suggesting that these two sections would make more sense structurally (from their perspective) if they were combined.

Beyond this, it is not possible for us to understand the exact purpose of the reviewer's suggestion, as we do not have access to the article you wrote, nor the rest of the review(s). In general, suggestions or comments in any review are best understood in context -- what is this reviewer's understanding of the paper? What is their likely background? What did they think of the paper overall; which sections did they like and dislike? Based on the answers to these questions you may get a better idea of why they are suggesting merging the sections, and how you might go about doing it effectively (or alternatively, how to respond to the reviewer if you disagree).

In addition to the meaning of this word in English, this may have been a mis-translation (a false friend from the referee's native language).

If the referee were Italian, for instance, they might have meant that you should reduce the length of these two sections.