Migration Multisite D7 to Single D8
There is more than one way to migrate Drupal content. Usually if you assign a migration from a database into a different database the assumption is that it's about migrating content into a new structure, without any additional content considerations.
As mentioned in the comment you should export/import the second site content rather than migrate.
You can look into Feeds module to import from a CSV, for example, and you can use Views to create a list of your content then export it into a CSV file. This isn't the only way to go about it, just one suggestion. Try to research the export/import ways of content in Drupal ;)