This is an old fic, I'm guessing pre-S5. I've searched but I don't remember either title nor author, and of course I didn't bookmark it. J2 are married and Jared wants to adopt. Jensen is reluctant, but agrees. In order to prove they'll be good parents, they take a class which requires them to take care of a baby mannequin for a period of several weeks. SPN is over, and Jensen isn't working. Jared has just landed a big movie with Vin Diesel, and they talk and Jared goes off to location shoot for weeks, leaving Jensen "holding the baby" mannequin. He manages on his own, discovering he would like being responsible for a kid, but resenting Jared for defaulting on their plan to adopt.

I'd be grateful for any help, and thanks.