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.
Errata — Chapter Approved: Main Rules Clarifications (v2.0): Because units can fight more than one enemy unit in a close combat, the attacks of the unit must be allocated before the dice are rolled. Attacks must therefore be allocated against enemy in base-to-base contact or within 2" if a model is not in base-to-base contact. If equal priorities exist then you may choose a target. Enemy characters (including unit upgrades such as Nobs) are always treated as being in a separate unit for the purposes of allocating attacks.
Errata — Chapter Approved: Main Rules Clarifications (v2.0): You may allocate attacks made by a character against a specific target model in an enemy unit if the enemy model is in base-to-base contact with him. These hits do not then carry through to hit other enemy models if the original target is killed.
Errata — Chapter Approved: Main Rules Clarifications (v2.0): You may allocate attacks against a model with different WS or T to the rest of the unit.