Don't believe him when he says he loves both of you. i had this with a guy, he kept saying he did but...and did this for a year and more. Really tore me up, then when I finally decided that Id had enough and he was just using me, and I told him that it wasn't going to work and nothing was going to happen, he accused me of using him...
Personally I don't think he would leave her, and hed just use you to make him feel good as in 'wahey I have 2 girls after me - aren't I just great!'