This might be a somewhat temporary problem and once they’re a little older, would be happy to share. I think as long as you make them both feel special (one cake each FOR SURE!) then they’ll be happy.

If possible, try having their parties on different days (maybe one before, one after, and keep the actual day for family). Make them both cakes, although they might be smaller than average.

Could you possibly do a joint birthday with family on their actual birthday and then one the week before and one the week after for friends? I think this will give them an inclusive birthday on the day and then their own special day each

Can you do separate things a weekend apart? Or maybe things like paddle pool, sprinkler mat, water balloons in summer. Or something like the zoo, aquarium, petting zoo

Whoa, how amazing.
I’d definitely go one theme if possibly – could do one of those cakes that is different each half to help out.
Games wise I would go things like the large foam blocks, bubbles, few other games aimed at the older children , pass the parcel may work.
Good luck and enjoy.

Maybe an agreed theme to assist with purchasing supplies.

Maybe a party with mixed games and party activities to cater for all ages and stages.

Oh wow on the same date I would just buy two indivdual cakes and if having a birthday party do 2 different themes

I have two cousins who share the same birthday and they celebrated it a week apart, so that both a day that they were the center of attention