I definitely would top him as he fucks her. The pressure on his prostate from my cock in his ass would produce a large volume of cum, hopefully resulting in her impregnation. And it's win-win because I'd get to blow my massive load inside his ass.
This is the method I use for my wife, I invite dozens of men almost every night to have sex with my wife, from 8pm to 2am they pass the role of inseminator in turn, usually in a few minutes it is done, about thirty seconds of penetration for one minute of ejaculation, during 6 uninterrupted hours of sexual intercourse.