For us, we had a similar opportunity with neighbors. They were lifelong friends of my wife and there had been some interaction between all of them in the past (the other couple didn't know I was in the know of the things that had occurred in the past). We had a huge hot tub. They came over often for meals, games, etc. There was always some comments made (I'm a flirt and always drop innuendo regardless of who we're hanging out with...all in fun). Knowing the other wife would be game immediately and the husband would give anything to do my wife, nothing ever went further than the flirty talk. My wife and I decided that dipping a toe in that pool could destroy the friendship. It just wasn't worth it.
My advice would be to join a site if you're interested in bringing someone else into your bedroom. While playing with the neighbor is easy and convenient, there is a high likelihood of four newbies messing things up. With the convenience, who is the first to become jealous when one spouse is working from home while the spouse of the other couple wanders over for lunch? Especially when your wife doesn't do oral and the neighbor's wife does. Is there an accusation of a morning BJ when you take too long to get the paper? What about your one-partner wife finding her inner sex goddess while riding the neighbor's large-then-your cock? She no longer seems content with your average ride as often as she used to and seems to only want sex when the neighbor's monster truck is involved. Simple friendly banter that occurs now will be looked on differently. This is especially true when one of the husbands can't get it up during an encounter (nerves are terrible for erections) while the other is a stallion. One of the men, maybe you or maybe him, could become infatuated with the other/new woman, like he's 17 again. These physical encounters will be an hour, a couple hours once, twice a week....but you live next to them 24 hours a day, every day. What happens when you give it a try and one of the four involved didn't enjoy it? Does the for sale sign go up? What happens when your kids see mom or dad is heavily flirting and touchy with the neighbor?
If you were both experienced LS couples, I'd say what the heck. You've been there before and know you can handle the sights and sounds. Newbies...no way.