What is the best way to organize a multi-website?

Hello folks :) Small question small answer here: What is the best way to organize a multi-website? We have a client who owns 2 spas and they did 2 different subdomains on wordpress for each spa. Then, they have a blog which is related to each of these subdomains. How should we construct site maps and seo for this kind of website? Any particular things to know? Many thanks for answers!