How to merge between 2 indexes in elasticsearch using c# ISearchResponse?

Question

I'm using elasticsearch c# ISearchResponse search. I want to get results from 2 indexes in 1 search. I want to get all users from the "user" index that their field "doc.Users_Email" is counting more than x times in the field "doc.SentMail_Recipients.recipients_email" from the "mail" index.

How can I do that? This example of a search request in my code:

 public List<string> Example()
        {
            ISearchResponse<object> result = _mainManager.Client.Search<object>(q => q
                .Index(ElasticIndex.User.ToString().ToLower())
                 .Query(c => c.Term("example")
                 .Size(10000));
            return _mainManager.GetIdsListResult(result);
        }

I tried this:

 ISearchResponse<object> result = _mainManager.Client.Search<object>(q => q
               .Index(new[] { "user", "mail" })
               .Size(3000)
               .Query(q1 => q1
                   .Nested(n => n
                       .Path("doc")
                       .Query(q2 => q2
                           .Terms(t => t
                               .Field("doc.Users_Email")
                           )
                       )
                   )
               )
               .Aggregations(a => a
                   .Nested("docs", n => n
                       .Path("doc")
                       .Aggregations(a1 => a1
                           .Terms("mails", t => t
                               .Field("doc.SentMail_Recipients.recipients_email")
                               .MinimumDocumentCount(10) // Set x as the minimum doc count
                           )
                       )
                   )
               )
           );

            List<string> userIds = _mainManager.GetIdsListResult(result);