public class Main
{
     public int Id { get; set;}
     public string Name { get; set; }
}

public MainLink 
{
     [Key]
     [Column(Order = 0)]
     public int MainId { get; set; }
     [ForeignKey("MainId")]
     public Main Main { get; set; }
     
     [Key] 
     [Column(Order = 1)]
     public int MainChildId { get; set; }
     [ForeignKey("MainChildId")]
     public Main MainChild { get; set; }
}