Is it permissible to wait to make a joint party for them both? Well, yes, if you really feel that it would be the right thing to do. The party is not what makes your daughter Bat Mitzvah - it is turning twelve years and a day. The same is true for your son except for him the date is one year later and both your daughter and your son will turn Bar/Bat Mitzvah with or without a ceremony and a party. Whilst I truly empathize with your desire to only have to make one party I do wonder if it would not be more significant for each of them if they had two separate parties. Each party can be more modest than one party (at the wrong time for your daughter) for both of them. This way they will each learn, in their own correct time, the importance of turning Bar and Bat Mitzvah.