Being married to a woman and her niece at the same time

Q: I want to marry my wife's niece who is 25 years old. I am 35 years old. I know that I cannot have both at the same time, but I have heard that one can marry another woman without giving divorce if he and his current wife have mutual consent not to have any physical relations. Please tell me if I can marry my wife's niece as I stopped all physical relationship with my current wife.