The second letter is being sent so that you can honestly say that every effort was made to inform him about the hearing.
I'm guessing you have never received a certified letter. If the recipient is not home, a card is left telling the recipient that they have a certified letter, when the post-office will attempt to redeliver, where they can call if they would like to pick it up, and most importantly, who it is from. Basically, if you ex- picks up his regular mail, he knows he had a certified letter, he knows who it is from and ignored it. So unless he had other concerns about being contacted by a government office (outstanding traffic citations, for example, or is being sued), he knows it is something to do with the custody and chose to ignore it. (He would also get notification that it was returned to sender after 3 attempts to deliver.)
Now, you don't mention if you both have attorneys. If so, he may have not picked up the letter if his attorney told him what it was and when the hearing was - he may have chosen not to make the effort. If there are no attorneys or mediators involved, he may very well not show, because how can he possibly know when the hearing is.
If he doesn't show, and later he tries to claim custody because "he didn't get his day in court", the record of these multiple attempts to contact him will show that he could have had it and chose to ignore it, which will greatly play in your favor.
There is clearly something about your ex that leads you to think he wouldn't show (lack of interest/communication with your children?), so perhaps this is just another way he is showing it.
If he doesn't show and the courts grant you full custody, he will get a letter stating that fact. He could still sue for visitation, but unless he is paid up in full on his child support, it isn't likely the courts will look favorably on him.
Hope this helps.
Kali
