Are you going to suggest to the hostess that refreshments be given to only one of your twins? Of course not.
Whatever you have to spend on a gift, divide it in half and buy a gift that each of your twins can take to the party. When you have twins, one of your tasks is to make certain everyone realizes that they are two individuals. Having the two of them take just one gift to a party flies in the face of their individuality. They will thank you for letting them be separate children, not one for the price of two.
Our parenting advice is given as suggestions only. We recommend you also consult your healthcare provider, and urge you to contact them immediately if your question is urgent or about a medical condition.