My parents and in-laws get along pretty well and live reasonably close, so they've seen each other a good bit. When we have one of the kids' birthday parties in their area, both sides of the family usually come (my parents usually host, as they have the bigger and nicer house with more room). Usually it's my parents, his parents, and his brother and family. My parents also sometimes invite them when they're having a party, holiday, or nice event, and we're in town. They don't invite them as much when we're at home (we live in another state), but they do occasionally. Also, my parents occasionally volunteer to babysit for my BIL/SIL's kids when they need help, or when my SIL is getting run down and needs a break (they have three little boys). I don't think it's exactly *usual*, but there's nobody toxic and both sides are pretty friendly and outgoing, so they tend to enjoy including each other. My parents are more likely to invite his than the other way around, but part of that is because his parents live in a house that doesn't have a lot of room for hosting, so they just don't host others all that often. When his parents were working as missionaries and lived elsewhere and came back for a visit for a month, my parents hosted them *and* us in their house so that we could spend time all together, which was really generous of them.
The extended families don't socialize as much, but his extended family will often invite my parents to larger events and holidays, like the 4th of July crab feast, Thanksgiving dinner, etc. My parents only go occasionally, as they are often the ones expected to host for *their* extended local family, which is just a few people, and they can't really drag everybody along but if they don't host then nobody will and the rest of their extended family won't have a get-together at all. They're more likely to go to things that aren't on a big holiday for that reason. But they do get invited and are welcomed if they come.
I don't think that this is necessarily the usual way that things are done, but I think that since both of our parents live in the same area (about 40 minutes apart) and are friendly, they try to make an effort, which I think is pretty cool. And it makes things a *lot* easier when we visit and want to see both families.