Single Sign-On for two CMS on the same domain. How to implement?

Question

User identification

Dear connoisseurs. The situation is this:
There is one site.ru domain , on which CMS Webasyst is installed. In the site.ru/wp directory, there is CMS WordPress with its own database.
There is a huge need to organize end-to-end authorization from the main CMS (Webasyst) to the second one (WordPress). Webasyst is used as the main shop site, while WordPress allows registered users to create their own posts.
In addition to authorization, the user must somehow "register" in WordPress. Those. unified system of registration and authorization. And all this should happen unnoticed by the user, as if he were working on a single portal.
How can it be implemented? Please note that I am not familiar with the topic of the database and SSO at all ...
Thanks in advance!