There will be 2 material masters. Each material master is represented by the material number (MATNR). This irrespective of the number of org units , plant etc it is extended to.OR irrespective to which view of the master it is extended to...
Each master can have different records. OR in other words extended to different master records relating to say different plants / sales orgs etc. But the material number remains same.
When you extend a material to say a new plant, you have to go to the 'CREATE" mode and create a new set of records for the material master with all extensions.. hope this helps...