create linked entities in EF based on automatically generated id

You could try this following after your call to SaveChanges:

myEntity2.X = myEntity1.Id;

Then call SaveChanges again. This doesn't really utilise the power of Entity Framework, however, which is in managing relationships between entities. If your class was defined something like this:

public class MyEntity
{
    [Key]
    public int Id { get; set; }

    [ForeignKey(nameof(RelatedEntity))]
    public int RelatedEntityId { get; set; }
    public MyEntity RelatedEntity { get; set; }
}

You could add your entities something like the following, and the Id/foreign key matching would be handled for you after calling SaveChanges:

myEntity1.RelatedEntity = myEntity2;

This is a fairly general solution, so if you'd like something more specific then you will need to include more details in your question.

You can read more about configuring Entity Framework relationships here.