Hi MAK,
If the materials are linked and the amount of material 'B' depends on the amount of material 'A' you can use free goods determination for material 'B'. In this case you could create the record in VBN1 and determine the relationship between material 'A' and free material 'B'. Thus the material 'B' is added to the sales order automatically after adding the required amount of material 'A'.
In case if the materials don't have a relation with each other, you could use different item category in sales order and the item for FOC material is not relevant for billing.
I hope it help.
KR
Sergey