"The problem is that I don't know how to tell my wife... Who is also pregnant." If he was in a poly relationship, it wouldn't be a problem. In fact, in that scenario, it might have been planned to happen that way. Sister wives, and whatnot. As it is, Captain Douchebag has bitten off more than he can chew, deserves to lose both women, and suffer in eternal loneliness, while hemorrhaging money in the form of two child support payments each month.

If he were in an open or poly relationship, he wouldn't worry about telling his wife that not only has he been cheating on her, but that he's knocked up two women at once.

You never know; he could be a brilliant dad. Being a two-timer does not automatically remove you from the right to be in your children's lives. If he wants to be in his kids' lives and he's competent enough to look after them, then let him. Of course, custody arrangements might be a little awkward; should he have both the kids on the same days? On the one hand, it could be a little awkward, but on the other hand, they're siblings, so it would be nice that they saw each other.

Whilst he deserves to lose both women and be forced to pay child support, I wouldn't bar him from seeing his children. For all we know, he could have really good paternal skills; plenty of cheating spouses are very good parents. As an example, I have a friend who is utterly appalling in relationships; he's cheated on multiple girlfriends and even has even admitted that he goes through women like some people go through tissues. Yeah, he's kind of an arse; something which he will gladly admit. Yet, to his two-year-old son (born from one of those flings), he is the best father a kid could ever ask for. I was surprised how well he took to fatherhood; he loves that kid with all his heart and would defend him to the death if necessary.