I sort of had this issue with my Spouse's other relationship when they were about three or four months into seeing each other. I asked for and received an agreement that we meet each other over dinner, which has not happened yet. Just having that agreement, and being friends with and chatting with Spouse's other partner on Facebook has alleviated those feelings within myself for the time being. Mind you - the metamour has never DONE anything to raise flags with me; they are very considerate and respectful toward both of us (spouse and me) and toward our relationship. I recognize that those feelings were mine and mine to deal with. By the same token, i WANT them to spend as much time together as all 3 of our schedules permit. I don't feel any jealousy; i just care that Spouse is happy and that they treat each other well.
I hope this helps you somewhat.