If a character accepts the challenge, move him into base contact with the challenger – after all, what good is the narrative of a challenge without the visual reality? If, for whatever reason, this is not possible, assume that the two models are in base contact (this might require a little gumption to work out, so it's best to move the model if you can). These two characters must direct all of their attacks against each other – they cannot be attacked by any other model for that round of close combat.