public class Grandpa
{
    public Guid Id { get; set; }
    public Father Robert { get; set; }
    public Father Jack { get; set; }
}

public class Father
{
    public Guid Id { get; set; }
    // I use this to reference Grandpa
    public Guid GrandpaId { get; set; }
    public List<Son> MySons { get; set; }
}

public class Son
{
    public Guid Id { get; set; }
    // I use this to reference Father
    public Guid FatherId { get; set; }
    public string Name { get; set; }
}

modelBuilder.Entity<Father>()
  .ToTable("Fathers");
modelBuilder.Entity<Son>()
  .ToTable("Sons");
modelBuilder.Entity<Grandpa>(g =>
{
  x.OwnsOne(y => y.Robert, z =>
  {
    z.OwnsMany(a => a.MySons, b =>
    {
      b.HasKey(k=>k.Id);
      b.ToTable("Sons");
    });
    z.ToTable("Fathers");
  });
  x.OwnsOne(y => y.Jack, z =>
  {
    z.OwnsMany(a => a.MySons, b =>
    {
      b.HasKey(k=>k.Id);
      b.ToTable("Sons");
    });
    z.ToTable("Fathers");
  });
});

Cannot use table 'Fathers' for entity type 'Grandpa.Father#Robert' since it is being used for entity type 'Grandpa.Father#Jack' and there is no relationship between their primary keys.