How to login in two different drupal sites at the same time


I’m trying to log in to two different Drupal installations in the same domain. For example, I have site1 (Drupal 8) and site2 (Drupal 7). If I login to site1, user will be automatically logged in to site2.
I have tried setting a session authentication in site2 with services module, and using it from site1 to log in to both sites at the same time but, the problem is, using the web service from site2 to log in just returns a user object, and doesn’t log in to the site itself.
Thanks in advance.

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