I'm not completely against it, but imagine how the child could possibly have to deal with. Should the kids fellow classmates find out, he could go into a world of hell. Unless, of course, he manages to hide it, or no one really cares if someone has two dads/moms. In some cases, the child could become confused, "Why do I have two dads, but everyone has a mom and a dad?" If the kid thinks this way and asks his parents, it could lead to a very confusing and emotional talk.
Someone find the flaws, I'm undecided.