Hi ,
There is no obvious solution to your requirement. You have to maintain as many BOM as finished goods. This function is available for Routing ,where you can assign multiple Finished goods to same routing
There are tools available to manage pain of maintaining multiple master data. Firstly, you can copy BOM of material A while creating BOM of material B. For changes , you can use Mass chnage function to add/delete/change qty of child material in all BOM in single click of mouse.
Hope it helps,
Jatin