First, just thought I'd point out that Approach B on this help page would not require any additional storage space and thus could be accomplished in your current subscription plan:
http://www.e-junkie.com/ej/help.package.htm
However, if you still don't want to take that approach, you might create a product-specific discount for both Product A and Product B, which would reduce the per-unit price of each product when the buyer's cart meets some Min. Item Quantity and/or Min. Item Total threshold(s). You may also need to disable "Let buyer edit quantity" in the product settings, so those thresholds could not be met by, say, just ordering 2 of the same item. If you leave the Discount Code field blank in the discounts' settings, the discount would be applied automatically, or you could configure both discounts with the same Discount Code, so the buyer only needs to enter that one code to apply both discounts in their cart.
