Mississippi State Incubation Troubleshooting
http://msucares.com/poultry/reproductions/trouble.html
Illinois Incubation troubleshooting
http://urbanext.illinois.edu/eggs/res24-00.html
You can read these as well as I can. While temperature and humidity are important there are several other things that can cause this type of problem. At least you opened the unhatched eggs so you know what time phase you are dealing with. That’s a huge benefit in trying to figure it out but often it’s still not easy.
Do you have the vents open during hatch? The chicks are living animals and need fresh air, whether still inside the porous or after hatch. I doubt this is your problem since you had two hatch and make it but I’ll mention it anyway.
Were the eggs positioned right, pointy side down, during incubation? If the eggs were laying on their side during incubation of course this does not apply, but incubating them upside down will often cause this problem. The chicks cannot position themselves to pip very easily.
My last thing to think about. Did you turn them regularly, especially during the first two weeks of incubation? Turning helps body parts form in the right places including internal organs. Turning helps stop the yolk or developing chick from coming into contact with the inside of that porous shell which can lead to its getting stuck or drying out. By two weeks body parts have formed and a membrane has developed to protect the chick from getting stuck to the inside of the shell, but it is pretty normal for chicks that go through this to die very late in incubation.
Good luck on figuring it out. That is very frustrating.