The old Part 23 required this:
a. The main wing flaps and related movable surfaces, as a system must:
(1) Be synchronized by mechanical connection; or
(2) Maintain synchronization so the event of an unsafe condition has been shown to be unlikely; or
b. The airplane should be shown to have safe flight characteristics with any combination of extreme positions of individual movable surfaces (mechanically interconnected surfaces are to be considered as a single surface).