Pay attention to the motives of this "separation". Why resort to this approach? If the answer is saving on another domain, then: a) this is not serious; b) and what kind of promotion can we talk about when saving on the main “name”, because this is equivalent (visually) to the site on a free hosting. And if the answer to this question is not connected with this reason, then we will resort to the experience of other sites with a similar approach.
For example, sites for personal blogs from Google, LiveJournal, etc., work precisely according to this principle, where each participant has his own subdomain created at the address of which contains his blog with a theme different from that of other participants. Nevertheless, search engines are quite well versed in what and where to look.