Hi lilyankh. I guess your husband and I are almost complete opposites but I can definitely pitch some insight into your situation. It seems we both have a curiosity for our wives having sex with their boyfriends. The only difference is that I have a somewhat jealous/competitive drive for my curiosity while your husband has a kinky drive for his curiosity. And despite knowing that my curiosity is natural and healthy, I still think that your husband has it easier even though you might not think so. If your bf is ok with it, ask your husband if he would like to watch you two even from a distance.
If you think he's too embarrassed to agree, let it go but don't be surprised if he decides to secretly watch from a distance because that's how I watched my wife and her bf the first time. I had only heard some things from my wife; I still wanted to see how he compared to me when he screwed her, how big is penis was, whether she enjoyed him more, etc. I was too intimidated and prideful to ask so I decided to take a peek one day. It wasn't easy at first because I found out he was screwing her better, he was bigger, and she did enjoy him more but it definitely quenched my curiosity. Even if those things might bother your husband, he will definitely be satisfied with watching you with your bf.
Most important thing is communication. See what he wants and what you can do to help him get it. If he doesn't want to tell you, just let him act accordingly and freely but with respect. Has he expressed his interest in watching you two?