DS1 (almost 3) and DS2 (almost 1) have their birthdays just shy of two months apart. This year, they are having separate parties and it feels like I have to work on the second one before the first one has even happened. It's doable, but it's also a bit of a faff/time consuming (and of course, costs money we could spend on more sensible things perhaps - though the money isn't really an issue).
So, in the years to come, WIBU to just do a joint party for them? For the sake of fairness I'd try to do it in the middle - so not on either one's actual birthday (a month late for DS1 and a month early for DS2).
My sister and I have birthdays a similar distance apart and I'm not sure I would have been impressed with joint parties - but I can't really remember my birthdays - so perhaps that's the answer - it just doesn't matter in the long run.
If I could go back in time I probably would have skipped DS1's second birthday, then I wouldn't do one for DS2 next year either - I've made a rod for my own back. 