According to the bottleneck theory a system is as fast as its slowest part.
If data flows through faster esram and ddr3, its one system, even if they handle different parts of the data (unless data sent to esram is calculated and sent to/ from a cpu/hdd/whatever outside the system, not taking away anything from the DDR) simply put if my left wheel can handle 200mph and the right 63mph, my car can handle 200 not 263. Thus on paper bandwidth is not a sum, if they take in and output data within the same system.
Two ddr3 modules do not equal a system with twice the bandwidth. Its more complicated than that in the bottleneck but its not a sum of speed.
Imagine you are on a train going 150mph. you can run x mph. On the train you can run, so relatively/locally for a brief period you will be travelling 150+x on paper. But the system will still have a max speed of 150 and will not reach the destination faster.
