public IReadOnlyList<Field> TargetFields {get; set;}

public async Task JoinTablesFromDifferentGeodatabases()
{
  await ArcGIS.Desktop.Framework.Threading.Tasks.QueuedTask.Run(() =>
  {
    using (Geodatabase sourceGeodatabase = new Geodatabase(new FileGeodatabaseConnectionPath(new Uri("Path \\ to \\Geodatabase \\ one"))))
    using (Geodatabase destinationGeodatabase = new Geodatabase(new FileGeodatabaseConnectionPath(new Uri("Path \\ to \\Geodatabase \\ two"))))
    using (Table sourceTable = sourceGeodatabase.OpenDataset<Table>("State"))
    using (Table destinationTable = destinationGeodatabase.OpenDataset<Table>("Cities"))
    {
      Field primaryKeyField = sourceTable.GetDefinition().GetFields().FirstOrDefault(field => field.Name.Equals("State.State_Abbreviation"));
      Field foreignKeyField = destinationTable.GetDefinition().GetFields().FirstOrDefault(field => field.Name.Equals("Cities.State"));

      VirtualRelationshipClassDescription virtualRelationshipClassDescription = new VirtualRelationshipClassDescription(primaryKeyField, foreignKeyField, RelationshipCardinality.OneToMany);

      using (RelationshipClass relationshipClass = sourceTable.RelateTo(destinationTable, virtualRelationshipClassDescription))
      {
        JoinDescription joinDescription = new JoinDescription(relationshipClass)
        {
          JoinDirection = JoinDirection.Forward,
          JoinType = JoinType.InnerJoin,
          TargetFields = sourceTable.GetDefinition().GetFields()
        };

        using (Join join = new Join(joinDescription))
        {
          Table joinedTable = join.GetJoinedTable();

          //Process the joined table. For example ..
          using (RowCursor cursor = joinedTable.Search())
          {
            while (cursor.MoveNext())
            {
              using (Row row = cursor.Current)
              {
                // Use Row
              }
            }
          }
        }
      }
    }
  });
}