@Ian,
the possible non-conformity of the Lion Air B737 Max with Part 25 is more related to 14 CFR 25.672 than 25.901.
Part 25.672 is specifically for stability augmentation systems.
The issue is not so much single or double point of failure but rather dedicated warning.
Originally Posted by Part 25.672
§ 25.672 Stability augmentation and automatic and power-operated systems.If the functioning of stability augmentation or other automatic or power-operated systems is necessary to show compliance with the flight characteristics requirements of this part, such systems must comply with
§ 25.671 and the following:
(a) A warning which is clearly distinguishable to the pilot under expected flight conditions without requiring his attention must be provided for any failure in the stability augmentation system or in any other automatic or power-operated system which could result in an unsafe condition if the pilot were not aware of the failure. Warning systems must not activate the control systems.
In the Lion Air accident, the second failure, i.e. not shutting off the electric trim, is linked to the ability of understanding the nature of the problem at stake and understanding that the stability augmentation system is failing.
Luc