A model can fight if its base is touching the base of an enemy model or if it is within 2" of an enemy model. Ordinarily we simply assume that the warrior is contributing his shots and blows to the swirling combat going on. This means we don't have to worry about whether individual models strike their respective opponents in base contact.
However, if a model is in base-to-base contact with opponents with varying Weapon Skill and Toughness characteristics it will be necessary to allocate attacks to different targets before rolling to hit. This is because the dice scores needed to hit and to wound them will be different. The model's attacks can be divided up as the player wishes, as long as he makes it clear before rolling to hit.