Fighting over toys is generally the starting point for aggression. Older twins may engage in less overt aggression, but tax their parents' patience with endless arguing. Arguing, within reasonable limits, has some important psychological functions. It also gives twins the unusual opportunity of struggling with an equal as opposed to a parent or order sibling who has more power in the family, or a younger sibling who will, naturally, be more protected by the parents. Parents should set limits on arguing such as no physical fights, no obscenities, or no fighting during dinner, but should refrain from forbidding or intervening in every argument.
Parents are the models for their children.
When it comes to limit setting, the best teaching is by example. Stick to the rules you set for your children and for yourself. Be clear, consistent and reassuring. Demonstrate through your behavior that anger is a real part of life that you try to deal with through words, not explosions. We'll talk about it when we have all cooled off.