I'm in a similar situation. In my case, my husband was the one who wanted to be poly and now we have a V situation where I have a girlfriend and he's not seeing anyone (although is working on that).
What I'm aware of in terms of work I have to do is making them both feel like they are valued and spending enough time with each of them without wanting either to feel neglected. It is a lot of work. I'm also a little resentful of my husband dating new women because I'm an introvert and it takes energy for me to have to get to know someone who may not even stick around for long.
What I try to look at is how it would be if he did get into some sort of stable arrangement. He'd be happier and I'd have to have less worry about him feeling abandoned when I was with my girlfriend. So all in all I know it's for the best even if getting to that point is difficult.