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
                            }
                        }
                    }
                }
            }
        }
    });
}