Medic would definitely push as hard as he could to try and shit, even if the majority of it is cum. Laughing at the gagging from the little outburst of both substances. Maybe he'd even go so far as to tell Scout the shit once came from the same dog whose cum Scout's licking out of his hole. After all, between the two of them Medic's considerably less pathetic. He's not the one back there, getting off on it no less. Scout tries to ignore the jeering and focus on getting it done already, even though he didn't exactly say no when Medic put him up to the task. Maybe they're already a thing and Medic's been making Scout do increasingly disgusting and/or otherwise humiliating things to appease him. And he's not hearing any no's at the offer to eat directly out of the dog's ass either. Maybe next time, Scout'll get to be the guard dog's bitch too.