Alternative item in BOM seems to fit your requirement.
Your material availability check needs to be active at the time or creation or release else you'd have to do it manually.
Add both A and B to the BOM and double click on item no. of A (component details) and in the Basic data tab you'll see "AltItemGroup" enter a group code (any two characters example AB) and hit enter a window will pop up there you'll need to enter more data like priority, strategy, probability. To enter values in these fields press F1. Enter the same "AltItemGroup" ( code AB ) for item B to group them together.
In your case you need to make the setting like below
For A
AltItemGroup AB Priority 1 Strategy 2 100% check Usage prob. 100
For B
AltItemGroup AB Priority 2 Strategy 2 100% check Usage prob. 0
You can buy and keep either A or B in stock when you do material availability check (manually or automatically) system will pick either A or B whichever has stock.
But the alernative item concept has more uses than just this, if you want to read up, I have provided some scenarios in this thread.
Normally good practice is to have only one number for a material that is identical.
If you need to stick to having more than one, you will need a special functionality called "full interchangeability".
This is not part of the standard SAP ERP delivery.
To get this functionality you need to activate DIMP business function (formerly known as industry solution discreet industries), and then use either the supersession chains from automotive or MRP-MPN sets from A&D. There are also some preconditions related.
You can also do this:
If your two materials only differ while purchasing but it is issued as one raw material to production then you can use manufacturer part number concept.
I implemented just by reading the following link
http://help.sap.com/saphelp_46c/helpdata/en/75/ee15c655c811d189900000e8322d00/frameset.htm
Active MPN in OMT0,
In transaction code S_ALR_87002201 create an MPN profile by checking relevant check boxes (make MPN mandatory for purchasing and other like info record update etc) assign it to the material master of internal material number (the raw material).
