We can offer a few observations.
--Make sure that both you and your spouse are on the same page--how far you want to take things. We wrote about this in an essay at this site a couple of years ago. It's more than "definitely interested in playing with. Here are our three points before going on the second date:
1. What are our fantasies? We are doing this for fun and with each other. We are going to blur the line between fantasy and reality. What does that look like?
2. What are our intentions? What is the reality of what we are going to do together? How much exploration do we plan?
3. What are our boundaries? Are we clear on what we will do together and what we expect of each other? What does approaching a limit look like, and what will we do if-and-when either of us gets there?
--Make that second date flexible and yet with a clear intention, for example..."How about dinner on Saturday? We were planning a light meal at our place. The hot tub is warmed up, too. What, if any, dietary restrictions or preferences do you have?" That last part turns out to be useful because you have made clear that you are concerned about and respectful of any restrictions or preferences they have in one domain of their lives, and you are likely to be just as respectful in other domains. Yes, this matters.
--Regardless of whether you are at a restaurant, or at home, or wherever, make the invitation clear AND give the other couple some time/space to decide. If we're at a restaurant, we'll make the invitation and then Mrs. FL will need to use the ladies' room and I will excuse myself to make a quick phone call. This gives the other couple the option of speaking with each other or the Mrs can follow Mrs. FL to the ladies' room for a private conversation. Either way it gives the other couple a bit of breathing room. If we're at home, while we're carrying dinner dishes from the table, we'll simply ask whether they would prefer dessert indoors or out on the deck by the hot tub. Again, it puts the ball in their court and allows them to make the decision. The general idea is to make your intention clear and yet give them a way to gracefully decline.
--Assuming things are progressing, this is a good time to ask about their restrictions/boundaries and to tell them yours.
--Patience is a virtue.