to clarify- what is a two family home? does it have separate levels for each family or something?
It would depend on the relationship to your parents- me, personally, no. If my parents were awesome and supportive and knew how to give me space, i would consider it, definitely. As long as you don't worry that it will strain your relationship with FH or your parents. I think your kids will appreciate the time with their grandparents later on in life. It would be great for saving money as well- just make sure that its not messy if either of the parties decides they want to sell or move out.
Seeing the posts about people saying their parents would live with them when they get old makes me laugh. My parents and I have an understanding. I told them long ago that they put me in daycare when I needed looking after and they can expect the same thing.
They even have a sticker on their fridge that says: "Be nice to your children, they'll choose your nursing home one day." Hee! If my parents want someone to take care of them, they better invest in LTC insurance or start sucking up to my brother because I'm out. I'll visit and bring stuffed animals (with the eyes removed so they can't swallow the small pieces, of course) and pies and all that jazz but that's it. I'm no Miranda.
Okay, that sounds harsh but I'm only half serious. I have a friend who runs assisted livings so I'll make sure they're in a good one. It'll be like Cocoon, only without the aliens.
I love my family, but heck no. My dad's side of the family does that...they all used to live next door to each other, and then my two aunts moved in with my grandparents. It's not fun.