Talk to them as a couple. It's not like 4 people meeting, it's more like one entity with two personalities meeting another one.

Everyone approaches this differently id imagine. For us i talk to the wife, hubby to the hubby if its messaging. We dont engage with the opposites outside of each other. Group messages and in person theres crossover but we honestly try and limit the individual contact and keep it more group. Its just not our dynamic to flirt with the other individually. Thats not others of course. Many prefer to almost have that 1 to 1 with the opposite partners. Since we dont try to develop a recurring relations its more about setting the boundaries and having some initial chemistry.

I'll be honest--it was an adjustment and I'm still pretty cautious. It seems there are all sort of minutia and micro rules to navigate so generally I let the behavior of the other couple (particularly the husband's) determine mine and focus on coming off confident and interested until I know what their dynamic is. I know I'm pretty comfortable with kissing and touching outside of playtime, so I try to base it on them rather than assume.

I've got other husbands that greet my wife with an open mouth kiss when we catch up, which despite myself having great chemistry with their wife, isn't a greeting I default to.

On the flipside, I've had instances at parties where I'm having an extended, very connected, play session with a wife--kissing, sucking fucking, having a grand old time. We'll separate a bit after play and run into each other again in another room and I'll go in for a kiss to indicate a familiarity and an interest in another round, but it will be met with awkwardness or even avoidance... I've played with them since without issue, but an unspoken boundary when you're out of the "safe zone"

There is a group chat dynamic, meet in public dynamic, playtime dynamic and post play/familiarity dynamic to navigate--hence I find it best to kind of go with the flow.