Big thing here is the membrane drying out. It'll become like a hard candy coating that they can't break. If it has been over 48 hours, I'd experimentally pull some of the shell off to see 1) if the blood vessels and such absorbed 2) if the membrane is as hard as it looks.
If the membrane is hard the chick is going to be trapped and may suffocate. So, go ahead and get them out of the shell. Take a warm damp (not wet) cloth and soften the membrane especially if it is blocking their nostrils and vent. You can help pull some of it off or just keep it moist and see if they can kick out if they aren't too exhausted.
Be very careful if you help get the membrane off, so you don't hurt them. Example: Make sure you don't tug on the umbilical cord instead of clipping it off, for instance.
I'm sure someone else will come along with some other tips.