To supplement to what is said by Sourabh,
Create an IBAU material with name you like as maintenance assembly (type IBAU) and create a material BOM through CS01 like below screen-shot.
Here 'ID_FAN_ASSEMBLY' is the IBAU material and the BOM has got 2 components with real material codes.
You can use this in the 'Construction type' field in the Structure tab of Equipment/FuncLocn master.
Also see this link.
http://scn.sap.com/thread/1619297
Jogeswara Rao K