Hi Theoff,
I don't think that it is possible to do that other than using Discount fixed amount charges and include those on corresponding product rate plans. Example:
On Product A create Product Rate Plan A100 which includes a charge of €100 for Product A and a Discount Product Rate Plan of type Fixed Amount. The Discount Amount should be the price of Product B - €75, e.g. Product B costs €12€, the discount amount should be €45. On this discount charge on Product A set "Apply Discount to" the corresponding charge model(s) of Product B and under "Include the following product charges" select Product B as product, the Product Rate Plan of Product B and the Product Rate Plan Charge of Product B (e.g. the charge that points out the €12€ charge of Product B).
Do the same thing for a charge of Product A with the amount of €150, i.e. create a discount charge, e.g. with the amount of €25, on the Product Rate Plan which includes the €150 charge and point out the same charge on Product B as above.
Then when you create the subscription and add the Product Rate Plan (€100 or €150) of Product A and the "standard" Product Rate Plan of Product B (€120) the discount fixed amount charge on the Product Rate Plan on Product A will kick in and reduce the price of Product B by €25 and €45 respectively and the totals on the invoice would align to what you expect, Product A = €100 and Product B = €75 > Total €175 or Product A = €150 and Product B = €95 > Total €245.
Let me know if this solves your issue or if you have any further questions
