You're rather close. You already use a label "opakowania" in the assembler. However, you haven't assigned this label yet to the product that is entering the assembler through channel 1 (the container in which the other products are packed.
I've added a simple formula on the Exit Trigger of the source names "Boxes", which will declare a label "opakowania" to each product leaving the source with either a value 1 or 2 (alternating).
i.opakowania := Mod(output(c), 2) + 1
Then the assembler takes either column 1 or 2 from the B.O.M.
I hope this helps you further,