Well, after writing a carefully casual email, we have a plan to meet the afore-mentioned couple at a neighborhood coffeehouse/bar type place later this week. We'll see how it goes.
And I didn't mean to start a with kids/without kids debate in the comments -- I can't possibly know what someone else's experience is like. From this side of things, I can say that it's often hard to become friends with people with kids, because they are busy doing all the kid type of things. I've been to too many parties and picnics filled with moms of 3 year olds who look at me blankly when I explain I'm a colleague of the host, not a playgroup friend. But one of my (not local) friends who does have kids complains that it's hard for them to meet couples they want to socialize with too. So I guess maybe it's just difficult for everyone of a certain age. Once you're out of school, it's harder to meet new people.
There's a hilarious How I Met Your Mother episode about this phenomenon, so it must be widespread enough to be reflected in sitcoms. Lily and Marshall get new across-the-hall neighbors and start "dating" them -- ordering takeout food, playing games, watching movies. And then the buzz wears off and they have to sneak around to get in and out of the apartment without the neighbors seeing them. Luckily for us, these new neighbors are several blocks away -- on one of my dog walking routes, but it's easy enough to walk another way if need be.