I do believe that nowadays they do make some 100 ton 2-bay cars. It is true that extremely dense commodities will usually be found in two bay cars such as sand and concrete. You will notice that Iron Ore is usually found in little 20 foot cars with one bay or in partially loaded hoppers because that material is so dense.
i would think that 2 , 2 bay cars would cost more to make/buy/ upkeep then 1 4 bay car?..........
if you have 2 two bay cars you have to buy and maintain twice as many wheels/trucks, and couplers. and twice as many "end sides" meaning that much more steel is needed.
the 4 bay you still only have 2 trucks and 2 couplers ands only 2 end sides. but can still hold the same as 2 two bays.
with all the extra trucks and couplers that just adds to the cost of man power to keep them in good shape,
thats just the way i look at it anyway.