K.L.
I had a friend who celebrated her childrens birthdays (6 & 1) on the same day last year. Their birthdays are a couple of weeks apart as well. She celebrated her son's first birthday with close friends and family from 12-1:30 and did cake around 1ish and then invited her daughters friends from 1:30-3:30 for a drop off friend party. The close friends and family ended up staying to hang out and celebrate both birthdays and each child had their own cake/birthday song and "celebrating me" time. The benefit is it's all done in one day, there were plenty of adults to help out with watching the kids and it felt like 2 separate parties.