I do not know how many rare magic items the players get. I say that you find level x item and they divide them up. It seems like they do not even pick them, or have one that I do not notice, so it has not been a problem yet.
I thought about a mechanic or formula that would add to an item that would make it better, or a rare. Rare items look like most get a property, an encounter, and a daily power. You can add these to a normal item to boost them giving one character a better item if the other character wanted to boost something else.
A lot of my players take those boots that let them stand as a free action, name is eluding me at the moment. That is a property and by itself is fairly cool. Ad an encounter power like being able to shift 1/2 your speed and a daily like jump your speed w/o an atk of oportunity. This would be a rare item and the other character with these boots can still stand as a free action, but he chose to take another item as his rare so he does not get these otehr things.