It's not possible to have a specific SiteAccess use a different search engine - they all seem to use the default one.

Steps to reproduce:
1. Create a new repository under ezplatform.repositories:

        repository_2:
            storage:
                engine: legacy
                connection: default
                config: {  }
            search:
                engine: AAAAAA
                connection: default

2. Configure admin_group to use it:

        admin_group:
            (...)
            repository: repository_2

3. Login into AdminUI as admin

Expected result:

• The configuration is taken into account and fails with:

   InvalidSearchEngine
  
  Invalid search engine 'AAAAA'. Could not find a service tagged as 'ezpublish.searchEngine' with alias 'AAAAA'.
  

  This is the behaviour on v2.

Actual result:
Admin panel loads without an issue and is using the default search engine - the configuration specified for repository_2 is ignored.