Generally from what I understand, you have the luggage allowance for the longest flight in your itinerary. ie. I flew CPT - JNB - IAD - YYZ, switching planes at each airport and I got 2x23kg for the whole trip even though I was on domestic flights for 2 parts of it. Best bet is to check with the airlines and see what they say. You might have to print out the luggage info and bring it to the airport to stop any conflicts from arising.