Near the end of the game, it might even be considered either an inversion or a distillation - if a higher-level player joins a lower-level group, they earn experience proportionate to their level - and retain some of their higher-level skills (and almost ALL of their enhancements) to compensate for the power drop.One of the biggest features in it is "Super-SideKicking", which means that everyone on the team is Sidekicked to either the team leader or the mission holder.
However, not leveling together means that the duo earns half as much XP as a normal solo character, meaning this is not for powerlevelling. If one character is logged out for three months, during which the other character gains twenty levels, then the first character comes back to find they've gained twenty levels. Issue 13, takes this a step further with a sort of permanent sidekicking ability: two players become, essentially, a duo, and they are always getting experience.
More directly, almost all experience earned by any character on a team will also be gained by each of the team's other members, as long as they're already roughly the same level.