This is definitely a tricky calculation or discount to implement. One way that I can think of doing this would be to have a calculation to identify every other item. You could then use the subtotal of the items that have this column value to discount by half.
The tricky part is coming up with the calculation. Since there is no easy way to determine what 'every other item' in the cart is you would need to come up with this logic. If you are up to the challenge I would suggest that you start out in the WA_eCart_Definition file in the WA_eCart folder. In here you can get a feel for how you can get at values and manipulate the data that is in the cart. Implementing this feature though would be completely custom.