我的想法是:
启程:
A (30kg / 2 x luggage) & B (no check-in luggage / hand-carry)
C & D (no check-in luggage / hand-carry)
C & D 共用一个check-in luggage,可是是搭A那里,所以A是用2个check-in luggage (自己的+C&D)
回程:
A (30kg luggage) & B (no check-in luggage / hand-carry)
C (30kg luggage) & D (no check-in luggage / hand-carry) > 共用luggage
回程的就不要省了,方便自己也方便人家

或许B也不小心买多了,那B可以搭A那里咯