Manufacture 'Material B' into 'Material A' (assign a BoM out of 'Material B' to 'Material A'), or 
create a new Product 'Material', assign two BoM - one manufacturing 'Material' out of 'Material A', the other manufacturing 'Material' out of 'Material B', or
just use multiple BoMs directly for the final Product.

Most meaningful way to go depends on how often 'Material A' and 'Material B' is used in other BoMs and how complex your BoMs are in general.