love is easy, and takes little space. Finding time and space to pursue it are not so easy. If you are stealing time and energy from one relationship to feed another, anger and hunger result. As katlaughing said, unless EVERYONE is agreed, it will mean only awkwardness and misery. There are people who do manage to live this way. If you and he and she might be that type, I suggest you read places like this, where people who cope with it discuss it at length. Good luck.

http://www.lovemore.com/

http://www.polyamory.org/