I've heard from two employees from the same airline that the US Federal Aviation Administration (FAA) limits commercial airline passengers to have no more than two carry-on items/bags/suitcases. Is that true? I wonder whether this is truly an FAA regulation, or some airline policy.
The contents of an FAA "Approved Carry-on Baggage Program" are described in this FAA document. Section 4 of this document requires, among other provisions, that carry-on egress from the aircraft not be impaired, that what a passenger can (and cannot) carry on board be specified, and that weight limits of luggage storage locations be observed.
No provision in this document creates an FAA-mandated limit on carry-on baggage quantity. But because each airline must describe carry-on baggage rules and policies that conform with FAA limits about cabin egress time and seat access, the effect of having a carry-on baggage program may cause the airline to limit, for example, how many carry-on bags each passenger may carry.
A quick scan through the FAA regulations shows (link):
§ 121.589 Carry-on baggage.
(a) No certificate holder may allow the boarding of carry-on baggage on an airplane unless each passenger's baggage has been scanned to control the size and amount carried on board in accordance with an approved carry-on baggage program in its operations specifications. In addition, no passenger may board an airplane if his/her carry-on baggage exceeds the baggage allowance prescribed in the carry-on baggage program in the certificate holder's operations specifications.
You would have to refer to the operations specifications for the airline in question to find out more about its approved carry-on baggage program.
Just as an example (or rather, a counter-example), Air France allow premium class passengers to have two carry-ons and one personal item. There is no mention of exceptions to or from the US.
There are probably many other such examples.