Originally Posted by
House
Baggage allowance will be that applying to the first flight you check-in for, unless this is ticketed as a single booking. Even then, if you are trying to use additional baggage allowance granted by Aadvantage status then this may not apply to the HKG-PVG flight.
The "longest sector" rule is
real, and is much easier to implement in this case than for those people flying ORD-PVG-<random place in China on different airline>.
ETA: The OP might need to be prepared to document the rule in question, but I'm guessing the check-in agents won't bat an eye.