you will need another cart column to mark the items that should get the discount. for example sakes, lets say the column is named:
buy1Get1
for items the discount applies to set the column value to 1 in the add to cart, and 0 for items the discount does not apply to
then a calcualtion to calculate the discount amount using an abs() condition:
buy1Get1Discount
abs([buy1Get1] == '1' && [Quantity] >= '2')?[Price]:0
Then create a discount to apply the discount:
triggers:
Subtotal for any column:
subtotal of column 'buy1Get1Discount' > 0
calculation:
subtotal of column 'buy1Get1Discount' plus 0
