Normally the most restrictive conditions, i.e. those of the lowest class leg, apply to the whole journey (possibly with the exception of cases where one leg doesn't have that class, e.g. a mixed F/J booking). Likely to have Y allowance the whole way though staff may be sympathetic at check in if you are only 1-2 kg over. I have never even come close to my baggage allowance so no experience of how flexible they are.