How to forecast for items with both dependent and independent demand. Do we have options for indepen

We have sub-assemblies used in other manufactured items that also have their own independent demand - sales. How should we forecast? One lump sum to consider both or can we keep the demands separate and only forecast the independent demand? if we use forecast consumption, will only the sales demand deduct from the forecast total or will it deduct the dependent demand as well?

Thanks,

Mark