Well, I've always played down to the 'lesser' level when we've travelled, Whenever we've only taken carry-on, we've only packed enough to meet the airline with the lowest weight, no matter what the others have been.

I would expect the 40kgs to be eliminated once you're booked in for Scoot economy. They are an economic airline, and could charge you for the overweight.

Who did you book with? Did you ask when you were booking your tickets?

#2 Like I have said the baggage allowance for each flight is usually shown very clearly in manage my booking and on the e-ticket

Did you search for business class when you booked and ended up with economy for one leg due to available aircraft type? When this has happened to me (on other airlines) I always got the larger allowance.

But as Alistair2 said, check your booking and it should tell you your baggage allowance.

Some airlines will quite specifically outline that the check in allowance is the lesser of the two amounts, however I have flown on a similar itinerary many times before with different airlines and every time, I have always been able to check in the greater amount and the bags were checked all the way through without any fuss.

Your experience may be different, and given your second leg is with a low cost carrier, I'd expect they will be strict about their check in allowance. However, as you're checking in with Singapore Airlines at the start of your journey, they may not say anything.

It is a shame we can not sell our unused baggage allowance I could make a fortune