Hi,
If I've understood the question correctly, it is relatively easy to implement this type of logic in a custom shipping module. In the method getQuote() of the shipping module, the order is passed in as a parameter. What you need to do is to go through the OrderProducts (an order has an array of order products) and figure out whether there are two items worth $50 or more. If so, you return a value of zero for shipping.
Hope that helps.
Ryan