How do you know if it is time to merge multiple sites or subdomains together? Here are a few ways to think about it…
1) Evaluate site audiences
How similar are the audiences for each site or subdomain?
If the different sites or subdomains target different countries, audiences, or languages, then separate domains/subdomains are often the right choice. Merging could make things more confusing for SEO and UX.
If the different sites or subdomains target the same audiences, then consider merging. For example, a blog might be located on a subdomain. A blog is usually tightly connected to the main site (in navigation, in internal links same brand, same visitors). Merging usually makes sense. It would consolidate SEO signals, helping the consolidated site rank better. Merging may also help UX by further integrating the sites.
2) Check user behavior
Do you see a lot of crossover traffic between the sites? For example, are people clicking back and forth between the different sites/subdomains? If so, that is a strong sign they should really just be one site. If there isn’t a lot of crossover traffic, then it might make more sense to keep the sites separated.
3) Content duplication
Sometimes companies create multiple sites for the same audience with slightly different messaging. For example, a micro site might be created for a new marketing campaign. Over time, those differences fade and the sites begin duplicating each other.
It can be easy to spot this in Google Search Console. You might see the different sites competing for the same keywords. Or, a lot of the pages from the sites may end up in Crawled, Not Indexed or Discovered, Not Indexed categories. I’ve recently seen sites with duplicate content get hit by the spam update. All of these are signs that it would be better to consolidate into one website to eliminate the duplication.
4) Business goals
It isn’t all about SEO. The multiple sites or subdomains could still serve a unique business or branding purpose. For example, one company sells the same products or services under two different brand names. If that is the case, merging isn’t really an option. However, from an SEO perspective, you’d want to find ways to differentiate the content on both sites as much as possible.
5) Technical considerations
Can you merge the sites technically? For example, a subdomain was created because the content on that subdomain uses a different CMS than the main domain. (A lot of blogs are on subdomains for this reason.) A reverse proxy ends up being a solution in many of these instances to resolve duplicate content.
In short, as with everything in SEO, there isn’t a one-size-fits-all answer. Sometimes it makes sense to have separate domains, either because of audiences, business objectives or technical considerations. In other cases, a merger is the better option to streamline UX and SEO.