you would need a column in the cart object for the shipping rate for each break point, for the example you gave, you would need breakPoint1 and breakPoint2
in the add to cart button for product1, set the breakPoint1 to 3 and breakPoint2 to 6.
for product2, set the breakPoint1 to 8 and breakPoint2 to 12
on the calculation tab, create a new calculation as:
shippingRate
(abs([Quantity] => 1 && [Quantity] <= 3)?[breakPoint1]:0) + (abs([Quantity] => 4 && [Quantity] <= 6)?[breakPoint2]:0)
then in the shipping tab create a shipping rule, use the trigger:
"Subtotal for any column"
if the subtotal of column "shippingRate" is ">" a value of "0"
Calculation:
"Based on multiple of column subtotal"
subtotal of column "shippingRate" "times" "1" "Plus" "0"