My co-worker - her DD's birthdays are 2 years and 2 weeks apart so they always do 1 party. her kids are 4 and 6 and they have never complained yet.
For family and at this age I doubt I would consider having 2 parties! Once they are in school I may do 2 parties for friends from school and 1 for family so they don't need to come twice. Makes 3 parties for you to host though!