Your approach doesn't look correct, If you are using 0material then it will from source return you single record always as it is MARA data which is material master (always single line for each material), How then you can have two lines for 0material (if you are loading it from standard material data source).
You should rather use 0MAT_PLANT (MARC data source), this will give you details as you want and will never overwrite data.
Thanks & Regard,
Mahen