Is a joint party weird?

So I have a question and don't know who to ask and I'm hoping your feedback will be helpful.

My parents have been married 32 years and are both turning 60 next year. Their birthdays are 4 weeks apart. My siblings and I would like to throw them a joint party to celebrate it maybe halfway in between the 2 dates. Is it weird that it won't be on their actual birthdays? What do you all think?