Going with the idea that your question was, "How do we (not should we) mention it???" we offer the following.
First, try to inititate a conversation on some issues critical to swinging, such as jealousy. You should be able to learn how they'd feel about the other being with someone else. Can they separate sex from love? Do they understand the difference? Look for red flags here and back off if they wave.
Never ask a question that can be answered "yes" or "no." Like you suffered through in college or high school, "essay" questions are the way to go.
Your friends don't have to make a decision, only to share their thoughts.
Wrong: "Would you two like to fuck us?"
Right: "How do y'all feel about swinging/group sex/intermarital sexual relations?"
Friend: "Why do you ask?"
Answer: "We have some friends (all of us here) who swing and they've invited us to, well, participate. (In this forum.)
We're kinda working on a decision... Really! We'd like some advice. What do y'all think? Can a marriage survive swinging?"
If they go negative on you, just thank them for their advice and swear you'll consider it in your decision. If they ask later what you decided, just tell them y'all decided to not go ahead with the idea. (the project of suducing them).
If the reaction is postitive. Talk! Talk! Talk!
Good luck in whatever direction y'all decide to go.
Alura