If I know I'm going to be in a duo lane then I almost always try to use a champ who won't need farm, and is still good just getting some gold/5 and a few basic items. No point in fighting for the CS with the other person and having two mediocre champs because of it. And because you always have to assume that everybody on your team is horrible at everything ever just assume they won't play a support and do the same for you. Relying on some random to play smartly and be a team player is just begging for problems.
Though if I see the other person is just terrible at last hitting I will start to take the farm myself, because its better going to an Alistar or Janna than nobody.
Like today I laned with a Jax as cow. At the 10 minute mark he had ~20 CS. I was trying to just let him take all the CS but he was just horrible at last hitting. So I started to take like all the farm and built AP cow to carry because Jax sure wasn't going to.
On the opposite side of the spectrum I laned with a Janna with my Corki, and the Janna kept raging the whole time because I was taking all the farm. She was a player who couldn't grasp that she doesn't need farm, where as I'm highly dependent on it. Even though I carried the team that game she was still complaining. Some players just don't get basic concepts :S