Hi,
For the co-products you have to set the co-product flag in the material master (MRP2 view) and also need to talk with your CO partner in order to define the way cost are distributed with the products (maybe rule of third per KG, or market cost, whatever bussiness want). You can set a default distribution rule (aka Appointment Structure) in the material master (MRP2 joint production). In every production order you can change the appointment structure based on the results of the process.
Arturo.