To be fair - having two people try to last hit in the same lane is almost universally a bad idea. You end up trying to last hit early to beat eachother, and end up with less farm in total than if one of you had priority. And if you need to decide whether Magnus or Naga should have priority, it should almost always be Naga. Magnus is better with low farm than Naga is.
That said, you're worthless as Magnus in that lane if you're not trying to get last hits. Next time, do as Zaphid says - go pull and kill shit in the jungle, then gank the lane with skewer when it pushes into your tower. It's suboptimal, but in AR you need to accept suboptimal lanes most of the time.