This is more a hypothetical question than one based on experience as I think it's likely to happen very rarely indeed. When using NDE on a fairly average front door, at least here in the UK, you might find a mortice deadbolt and a nightlatch with a rim cylinder.
The technique would obviously be to pick the deadbolt and then pick the spring loaded rim cylinder.
What happens if there are two rim cylinders with sprung nightlatches? You can't pick both at once and you need to maintain tension on the first one that has been picked so you can pick the second. This of course may leave you a tension tool down.
It's possible that with a letterbox tool you could drop or lift the snib to keep the first lock open or perhaps use mica.
If you can't use either of these methods, you're probably going to have to rely on someone else to hold the first lock open while you pick the second.
Has anyone ever encoutered a door with two nightlatches? Any thoughts on other (non destructive!) ways to get round the problem?