I have actually experienced this from your partners side of things. Because of the fact that the other person that was involved was a very close freind to both of us, it was kind of a double betrayal for me and nearly ended my marriage. Although it was hard, my husband and I were able to work through it and we have stayed freinds with the other person. It did not take me long to forgive both of them for what had happened. I love both my husband and my freind and understood how things got to where they were, but it took some time for me to be comfortable with the idea that he would like to be with her again.
I thought I would let you know how we were able to get through it in case it might help you, but I will have to admit the process took a few years. I think firstly because the other person and I are freinds so there is not this unknown person with unknown motives involved helped alot for one. I do not know if you can get together with your two men just for something social from time to time, but that could help them understand the situation better.
Also, my husband spends alot of time reassuring me how much he loves me and how his feelings for others take nothing away from that. We have both seen others for a couple of years and have worked very hard to stay in touch with eachothers feelings and not let things get out of hand. He never asked me if he could get back together with the first girlfreind ever, but from time to time he would explain to me why he felt so close to her and how he wished he had approached the situation differently so that I did not experience so much pain from it. I know that he has never had that kind of connection with another person that he has dated as he had with the first girlfreind. I was the one that finally suggested that now I that now that I have grown much in my understanding, I would be comfortable if he wanted to revisit that relationship now in the open instead of in secret.
Hope it helps!