Only from the perspective of the 'inferior' player - 'co-pilot' seems fine for competitive multiplayer, since as far as other players are concerned you could have been performing better, if you were only the better of the two of you, 'unimpeded' by the one being assisted.
Many games have "assist modes" of some kind. Sometimes as part of a difficulty setting, sometimes you have fine-grained controls, especially in racing games.
Usually, when these modes are enabled, there is some indication that they are used, and you can't enter ladders with them, to prevent cheating.