When I saw the title of your topic I thought what interesting timing
Kael's b-day is tomorrow (the 12th) and since I'm also going to be 12 days overdue tomorrow, my doc says we must induce. So, I've been trying to deal with some superficial feelings about how my kids are going to feel growing up with the same birthday.
Anyway, in answer to your actual question, I actually enjoy that many of the families we invite to birthdays now have a lot of kids. It allows for us to stay sort of low key, only inviting a few different families, but still have a nice number of kids there for the birthday person to play with.
Previously we have done a big party every year for each kid (with 2 instances of a cancelled party due to horrific behavior), but I am finding that the kids do seem really spoiled about their birthdays. We took our 3 year old out chuck e cheese this year with just our family and a cake, plus presents, and really it seemed fine to me and so much cheaper. I have been thinking that I will be doing more of that, since it will really be a stretch for us to afford four big parties a year. Not sure what I will do for the two with the shared birthday now...I feel like I ought to do something special since they won't get their own day.