PP here. I usually have a party on Saturday and Sunday on the same weekend.

I don't have twins but I have 2 boys with birthdays 1 day apart. They have separate parties. I invite some good friends to both parties but they have different friend lists since they are in different grades. Surprisingly most of our family friends come to both parties.

I would not do separate parties if they had the exact same guest list.

I have twin boys as well as a third boy a little younger whose birthday is within a month of the twins. I have had a combined venue party for all 3 of them. Those venues are expensive and I (and many others) cannot afford to throw 3 parties like that per year. However, I can have each child invite 6 friends and get the whole thing over with all at once in a 3 for 1 deal. Fortunately, they are all boys whose interests mesh well.