Sometimes a particular model will find itself in base contact with two different kinds of enemy, perhaps warriors from two different units, or even a rank-and-file warrior and a character.
If a model is touching enemies with different characteristic profiles, for two or more characters or units with the same characteristic profiles, it can choose which one to attack when its turn to strike comes (before any dice are rolled).
For example, if faced with an enemy character and an enemy warrior, you might decide to attack the warrior because he is easier to kill, or you could take a more heroic path instead and attempt to slay the enemy character.
Similarly, if a model has more than 1 Attack, it can divide its attacks as the player wishes. Only the most skilled, experienced or formidable warriors have more than a single Attack on their profile, and it seems sensible that such fighters would have the wits to strike at more than one foe should the opportunity present itself.
There's no right or wrong in such a decision – just make sure you declare your intentions to the opponent before any dice are rolled.