I believe that as long as your bag is tagged through to Canada/US, the 2*23kg applies.
If you are making a stopover in AMS your bags will be tagged to AMS only, I'm not sure if then the 1*20 kg would then apply.
And sometimes on the European sectors operated by a Cityhopper (not LHR) they are difficult with handluggage, especially if it's a trolley that does not fit in the overhead bin of Fokker 50/70. They won't charge you extra but may put it in the hold.