It's been a while due to scheduling and distance but my GF and her BF and I hang out and it is a lovely time. In fact, my second date with her was dinner and convo with the three of us and we've shared several more times of just being together sharing movies, music and personal narratives. Along with the sometimes awkward negotiations around sleeping arrangements (LOL).
I have found that sharing space and events is a wonderful way of helping to keep the other love from becoming some kind of monster onto whom we can project some of the almost inevitable feelings of jealousy, doubt, or whatever upon.
And the beauty is that BF and I have a genuine liking and respect for each other. That just adds to the beauty and depth of the connection.